The general consensus of literature on the psychology of "extremism", is there is no "one" character or personality type who gets involved.
I'm obviously not very comfortable with it but the label "extremist" does apply to me to some extent, if only because of where my sympathies lie well outside the mainstream. I was in the Communist Party very breifly but actually is was at the very very end of my interest in communism when I was already begining to suspect there was a problem with it. I joined because I knew I'd regret not doing it if I didn't. it was one of those mistakes that is simply too big not to make and was going to be life-changing whatever it was. the impression I got of them isn't really deatiled but they did not come accross as the kind of "dedicated" people I wanted to be around. it felt half-hearted, ametuer and potentially rather dogmatic.
Right at the begining, I had alot of personal problems and the onset of depression made me very vulnerable. I had an intrest in Communism stemming back several years but it was sort of an "attraction" rather than anything deeper. But, at that point of human vulnerability when my life seemed to be falling apart, I was really susceptable to the need for some "over-arching" framework that would make sense out of life. It was like "finding religion" and the only reason I didn't become religious at that point was for philosophical reasons (i.e. being a materialist). I felt "safe" as now I had a system which could provide me an answer for everything. the ideal I wanted to live up to was the "dedicated" type but "confusion" and "boredom" played a role as well. its hard to describe how fulfilling that sense of "dedication" is and what it means to have some sense of meaning, purpose and drive in life- particuarly when nothing else seems to "do it". its still an ideal I aspire to but whilst accepting that a certian level of selfishness is healthy and balanced. overall, mental illness plus an "extremist" political ideology was not a good mix and my first two years with an indepth interest in it, I was essentally at "war" with my parents because of how robbed I felt about being denied a fulfilling/happy life. This is the point where you could legitimately call it "hate". What I didn't realise was I was bisexual, sexually repressed and frustrated and that fed into it. my parents were psychologically abusive in many ways so being "angry" was overall more healthy than keeping it in. there was a legitimate reason for me to feel that angry even if I wasn't conscious of it but developing the self-respect ot accpet that took time. As I started to read up on marxist inspired psychologists (Erich Fromm/Wilhelm Reich being my favorites) I become more open and accepting of my sexuality and more intrested in ethics, my "fanatical" phase passed. [Eric Hoffers Book "The True Believer" is a conservative angle which is extremely insightful into fanatcism, but doesn't offer an explanation as to why people go through it beyond vague references to "frustration".]
I started to question communism much more deeply and seriously think about the "ethical" problems of violence and look deeper into the ideology to see what it said on those subjects. the answer was "very little" to the point where "nothing" would be accurate because of how heavily the ideology was driven by expediency. they never really treated "ethics" as a distinct discipline as what was ethical was whatever was necessary to wage the "class struggle" against the "enemies of the people". this is when my "beliefs" started to unravel. Any justification I may have had for it simply fell apart it took a considerable emotional and psychological toll because of how disturbing it all was (and it has only really settled down within the past month or so). there were several times when I tried very consciously and deliberately to walk away from it, trying to find beliefs that could fill the "void". I looked into Ayn Rands Objectivism, Anarcho-Capitalism, read the Road to Serfdom and Freidman's Capitalism and Freedom as possible alternatives. They didn't really stand up to examination but were incredably useful to think over things and see the world a different way. I did read The Doctorine of Fascism and got a copy of the Hitler Youth handbook to see what the similarites were between Communism and the far-right (the answer is there are fairly superfical similarities but they both have a strong conviction in the necessity of political violence). the reason I reverted back to "Marxism-Leninism" recently was because, after trying to change, I realised it was a form of self-harm so I had to find some level of "acceptence" that this is who I am. I am still very much a product of this experience and it will take years for me to know where I go after this. So the label will stick for quite a long time, as I still accept the basic premises of the philosophy even if my reaction to the politics is being appauled by it's cruelty. But there is a tacit admission that for the entirity of the whole time, I never really understood what Communism was, nor really respected what it meant to people who lived through it (both for the perpetrators and the victims). there was always some level of "denial" that came from being repressed and unable to be emotionally open about why I "enjoyed" it in a perverse kind of way.
I have been on Revleft.com which is the forum for the far left. I did look up polls to figure out what the membership was like. something like 80/90% were under 35. about 60/50% were atheist. 30% were gay or bisexual. very few voted, very few were members of a political party. because of that age group, there was a very large proportion of people who had a history of depression or still had it (myself included). there were also a fair number of people who had other mental disorders but they were all very low-level and common disorder not the "big" ones that suggested something really wrong. I left because of the second sucicide attempt and was left pretty sure there would be more. the atmosphere wasn't very healthy and troll-like, but I do miss it occassionally as being on the margins of mainstream opinion is very lonely and isolating. I am still happier and even healthier this way, but there are costs to it. so I wouldn't reccommend it to anyone. if anything, I'd be the one trying to talk people out of being communist now.