good doesn't prevail, as with the real world, D&D world is a harsh, dangerous and uncaring place, I have read monster manuals and seen a lot more of evil monsters than good ones, so yeah, bad guys are, by rule, stronger and/or more than good guys.
and that's why is such an event when good guys win against bad guys, that's why they are called "heroes" because odds, rationality, logic and common sense said they were gonna lose and die horribly, but, they lived to tell of their victory (or, at least, someone lived to tell of their victory and horrible death for the cause).
The thing is, while good doesn't prevail, evil doesn't either, not openly, not the theatrical, black-plate-wearing, undead-rising, purity-defyling and fall-of-all-righteousness evil. But the little evil that most people do everyday, indifference and lazyness.
Back to wether it is or not evil to rise an army of shambling corpses, that depends only on the DM look at the value of life, in my campaigns, life is a valuable, unique thing, something that, when lost, can never be recovered, and making life falsifications (read: undead) is quite an act of being an evil/insane dick (and yes, for those who really wanna know, resurrection and other spells that actually bring back the dead are banned)
my two copper pieces