There’s no word for “not a murderer.” “Murderer” is the word we use to describe a person that kills others in cold blood, but we don’t have a word specifically for someone who doesn’t do that. That’s a good thing! Having a word for something generally means you need a word for that thing, and I like living in a world where we don’t have so few non-murderers that we need a specific word to differentiate them from everyone else.
The broader point is that you shouldn’t label or define yourself by what you don’t do. Take veganism, for example. I support anyone making ethically-driven choices along their own value lines. That part is fine. But if your moral system gives animals enough moral weight to be worthy of not being murdered, not murdering them isn’t really doing anything special. It’s the same as me not murdering humans. It’s literally the least I can do; it’s not praiseworthy by itself.
If you think lots of people are doing something that contributes to a problem, then not contributing to that problem isn’t good; it’s just neutral. You’d have to identify the problem itself and take active steps towards solving it in order to be doing “good.” In other words, good is defined by the positive action, not the negative space between actions.
If you’re not part of the solution, then (maybe) you’re part of the problem. But not being part of the problem doesn’t make you part of the solution. That’s it’s own thing.