Genes can make you more predisposed towards violent behaviour (via hormone expression levels and stuff like that), but the environment is the set of factors controlling the expression of these genes, to put it simply. It seems like nowadays, we like linking everything to genes, but we tend to forget that genes are like a cake recipe - nobody ever follows those word for word, because they might want to add a bit more of their favourite ingredient, or maybe they don't have something and put an ersatz in, or... Yeah, in this sense your genetic code is like a cake recipe, only more so.
EDIT: Didn't really put a conclusion in there... So: a "violence gene" will not make you join a gang, or kill someone... It might make you more predisposed towards violence, but ultimately, it's the environment which decides.