What is good and evil?

My apology in advance for grammar and spelling, I am a bit drunk... but wanted to ask such question for a long time, so it makes no difference.

What is good? Hmm... There are religions who deal with such questions. I intend to visit some priests but before I do... maybe you can give me some hint. Par instance, Christianity tells "you don't kill". Fine. But there are "heroes" in our history who killed so many yet we perceive them with greatest respect. Napoleon. Caesar. Alexander the Great. The other view could be that by conquering and uniting many lands these people prevented many more civil or insignificant wars that could cost more lives then their own ambitions. Moreover, these people brought culture, knowledge by spreading their own culture. As a matter of fact I would agree with such argument but this is my pure subjective opinion.

So... If I (we) really can't decide who's murdered and who's conqueror, what good means anyway?