Could you elaborate and explain further for me please. What do you mean? Could you give some examples and contrasting examples to make it clear what you are meaning and talking about? Also, what you think the best lifestyle and diet is and how to avoid everything you think is bad to do or eat, and what those things are and why they are bad to do them or eat them.
By being alive, one is continually inadvertently and/or deliberately killing or contributing to killing. All things are inter-connected. At the human society level it is easy to see this. My clothes, my accommodation, my means of transport, where I live, the energy I consume (its source), what I eat, the products I buy. You don't need to dig too deeply to find where in all of this one is "contributing" to death (buying almonds from California? I'm killing bees). So it is not just about diet, but that is one important facet. For example, reduction in meat consumption is a major way to counteract global warming. One cannot escape being a cog in all of this. All one can do, if one wishes, is to become better informed about such matters and then make our "lifestyle" decisions based on that; what we are comfortable doing, what sits best (or least worst) with our consciences.
Given what I've just said, I think a person has to find (assuming they are looking to make changes) what fits best for them. Where you live and your personal circumstances may have an impact on the choices you can make. The "best" for me won't necessarily be the best for you. And the "best" for me isn't how I always am, mainly because I'm fallible and hence hypocritical. But I sleep OK. "Lifestyle" is a rather large notion so maybe I'll just say a bit about diet. I consider some form of vegetarian diet (lacto, ovo, pescetarian, vegan, whatever) to be the healthiest one, I believe it is the most compassionate choice and it is the best choice for the planet (biosphere, global warming blah blah blah). My body does not need meat, I am an omnivore. I have followed a lacto-ovo vegetarian diet for nearly 40 years. In the last couple of years I've switched to a mostly vegan diet, as a result of seeing what the dairy and egg industries involve (duh). Whatever diet we follow, we should of course check we are getting all the nutrients we need, not too much of the things we shouldn't have too much of and none of the stuff we don't want (Will you be having steroids in your meat?). Also, check the sources of foodstuffs - different countries have different legal standards and rules for various matters (eg permitted drug use, preparation methods, animal welfare etc).