Stalin was more deadly, but was he more ruthless? 2 people can be equally ruthless but create different outcomes due to other factors.
Stalin killed more people perhaps due to the circumstance of taking over a divided party where many people didn't support him. Perhaps personal temperament or paranoia. Probably due to these and numerous other factors.
But, Lenin was pretty explicit that whatever it took to further the revolution and defeat its enemies was morally justifiable.
This is what I mean by ruthless.
I don't see a major distinction here, personally, because many political or ruling figures throughout history have considered it morally justifiable to do anything to pursue their goals. This could range from maintaining or consolidating power (as in political purges and systematic massacring of opponents) to "perfecting" society (as in Marxism-Leninism). The French Revolution included a variant of this, and so did the Russian Revolution.
Not all historical figures who practiced this "end justifies the means" approach stated it as explicitly as Lenin did, but it's clear from the actions of many of them that they had little or no hesitation in doing so. Marxism-Leninism ended up being an especially harmful manifestation of this line of thinking because of how much traction it gained during the 20th century, although its underlying concept of "violence for the greater good" wasn't the first or last of its ideological kind. It just stressed it in more explicit terms than a lot of similarly utopian ideologies.
Also, even someone who claims readiness to engage in infinite violence to reach an ideal state of social harmony will only live 70-80 years on average, unless they're overthrown or killed. It doesn't make a practical difference whether such a leader believes in utopian violence, which would be endless because society will obviously never become perfect, or believes in violence only insofar as it serves his own reign. In the context of our average lifespan as humans, "infinite" and "lasting through a leader's whole life (or close to it)" are practically the same.
The main problem, in my opinion, comes from continuity of such harmful ideologies across successive regimes. There are many murderous tyrants in history, but having a series of them in succession as was the case with the USSR results in unmatched amounts of death and suffering. In that context, I wonder whether the magnitude of destruction caused by an ideology is more of a function of its adoption of a principle like "the end justifies the means" or more due to the amount of dedication its spearheads have throughout successive regimes or political parties (e.g., in the USSR).
I do draw a line between utopian violence and 'normal' violence.
If the Whites had won there would have been violent reprisals and oppression certainly.
The use of violence to accelerate progress by cleansing and purifying a society removes constraints though. Normal violence has a tangible and achievable goal, for example eradicate enough communists to end resistance and use enough force to keep dissent at manageable levels.
When the goal is human perfection and an end to suffering, anything is acceptable because the end justifies the means. Forget the sentimental attachment to the idea an individual human life has value, that is just bourgeoise weakness. The greater good is all that matters and the utopian upside can support any amount of short term harms.
When the goal is impossible but you are justified in using unlimited violence to achieve it, this is something different. It justifies unlimited, and generally increasing violence until the goal is abandoned for something that can actually be achieved.
Utopian totalitarianism is thus qualitatively different from normal autocracy, even very violent autocracy.
It seems to me that there are too many motives behind violence for there to be a meaningful grouping of "normal" violence under one label, though. For instance, would religious violence and imperialist violence fall within the category of being utopian, or would they be "normal"? After all, a lot of religious violence in history was based on the notion of spreading "truth" or pursuing a perceived greater cause. Similarly, following the Enlightenment, "rationalism" partially underpinned the idea of "civilizing" other cultures via colonialism and conquest, and this served as one of the motives behind genocides and "white man's burden" thinking.
Imperialist violence in particular rarely relented when empires sought to expand their territories or "civilize" people. Sometimes it would only notably decrease in frequency or intensity after genocides or subjugation of entire peoples. Each empire was different from the others, of course, so I'm speaking very broadly here, but hopefully my point is clear about why, in some cases, I find it extremely difficult to draw a distinction between utopian violence and "normal" violence. The idea of normality requires a common or standard approach or MO, and I think the motives behind many instances of violence in history are too variable and context-specific to establish such normality.
Of course different peoples will have different views, but in terms of total impact Lenin was definitely global.
Most people don’t make all that much difference. Replace any British official and the outcome is similar. The same geopolitical forces exist, the strategic importance of Egypt and the Suez Canal to major powers still exists.
Lenin was someone who was personally important, as someone like Hitler was.
Without Lenin there is a decent chance the Russian history plays out very differently and there is no Communist Russia. Far from a certainty, but a realistic possibility. In that case you probably wouldn't get Hitler and the Nazis (as anti-Bolshevism was an important factor in their ideology and rise to poser), probably no WW2 (although a smaller scale war may have happened), no Stalin, no Mao, Pol Pot, etc. as they relied on Soviet support, no Cold War which impacted much of the world.
It's always speculative and subjective, but imo if there were no WW2 or 20th C totalitarianism, most outcomes are likely to be better.
I'm aware that Lenin's impact was global; I'm just saying that many other ruthless leaders were global in their impact as well. In that regard, Lenin wasn't different from many other historical figures.
There's also the question of whether someone like Lenin would have still appeared in one society or another. I'm a firm believer that every single person is a product of human nature, socioeconomic circumstances, upbringing, exposure to certain ideas or beliefs, etc. If we attribute the roots of all of the issues you listed to Lenin, why can't we go back a step further and say that if the Russian Empire hadn't been unjust, Lenin would have never developed Marxism-Leninism or managed to hold power in the first place? His ideology gained traction partially as a result of the perceived injustices imposed by previous rulers, so it seems arguable that we can kick the can down the road and attribute later issues to the Russian Empire's actions as well, even if partially.
Most of the other issues you listed also involved multiple actors of arguably equal or at least very similar levels of influence and power, and it's impossible to know whether the absence of Lenin would have really prevented some of them. Would Hitler and the Nazis have simply found another way to rise to power? Would the likes of Stalin, Mao, and Pol Pot have still formed their own violent utopian ideologies and imposed them on millions of people?
Also, the Cold War was an extremely mutual affair: since the US pursued it with no less vigor than the USSR did, I don't think I would rule out the possibility of a similar power struggle with another global superpower even if the USSR had never existed. Some similar tensions are now happening between the US and China, and competition for hegemony between global powers far predates both the USSR and the US anyway.
Contemplating these historical scenarios and hypotheticals reminds me of an argument I have encountered among a lot of "New Atheist" circles: "if we could go back in time and prevent Christianity and Islam from being founded, the world would be such a better place." But this seems to me extremely speculative and unevidenced. There's no evidence that humanity wouldn't come up with similar ideologies or religions, especially considering that a lot of the appeal within the major world religions is rooted in human nature (e.g., desire for community, meaning, connection with a guiding influence, etc.).
There's also no way to tell with any degree of meaningful certainty how history would have shaped up without these major influences on human history. Maybe it would be similar to now, maybe it would be worse, and maybe it would be better (and as I said earlier, even "worse" and "better" are highly subjective and variable depending on who you ask and where). There are too many interconnected variables for a change in one of them to have yielded predictable or deterministic results.
I lean toward the position that history in such a hypothetical, different timeline would be at least broadly similar to our own just because human nature would be the same, and ultimately, much of our history is shaped by our nature and the fact that humans are primarily not rational actors. We have created all of the ideologies that exist today; I don't see why we wouldn't create similar ones in a different historical timeline.