I believe in "good" and "evil" only as frameworks erected by a society of people to teach which things benefit the society and which don't. It's a perfectly sensible thing for a society to do, since then it can praise (and encourage) acts beneficial to the society, and condemn (and discourage) acts that are selfish, unlawful, or otherwise detrimental to the group.
If you're asking me about "good" and "evil" as cosmic forces vying against each other and trying to influence the galaxy by proxy (i.e. through the acts of humans), then I'm afraid I'll have to say you're full of bollocks. There are no cosmic forces, one good, i.e. selfless, compassionate etcetera, that rewards you through karmic means, and one evil, i.e. selfish, ruthless etcetera, that will punish you at the end of your life, even though it benefits you during it. We make decisions based on what we were taught about good and evil, and based on the positive or negative consequences they might have for us.
As the Witcher so aptly puts it: there is no good and evil. Only decisions and consequences.