Let's say a murderer. Maybe he's just killing people so there are not so many hostile "species" left to harm his family (therefore his genes will get passed on more likely)?
Could be a case made for it in some situations.
A rapist, maybe he's just increasing his chances of passing on his genes?
Yes, this is true.
A musician, musicians are known to to make people happier, a happy family is more likely to not kill each other and are also known to be more sexually active.
Indeed, an inate musical talent can be evolutionarily advantage. It might be nothing to do with physical genes but cultural memes of course.
My point is (if you didn't get it), anything can be ascribed to having an evolutionary purpose, but it just may not be that. Or can it?
Indeed. The point that kuresu and others (incl myself) that some things which seem on the face of it to run counter to the concept of selfish genes or 'survival of the fittest' (homosexuals would seem to pass on less genes than heterosexuals, on average) can still survive because of kin with similar genes who do reproduce more.
Is everything we do related to passing on our genes?
That's kind of the conclusion Freud came to. However, I think everything we do is rooted in genes, but that culture has a bigger impact on our actions.