OK, first off, D&D is a fantasy game!!! It is not based on reality! Obviously, for the sake of having fun, they had to make some kind of rules, but for the most part, you should just let your imagination run wild! Who says you can't be a Chaotic Evil Gnome Barbarian? Who says a dwarf can't be a necromancer? Let the PC's define the characters story and style. You make the game happen.
If soemone wants to play a Lawfull Good Necromancer, why not? make them explain to you how the character veiws death, and how it constitutes a lawful good alignment. If you believe the character should be Lawfull Neutral, tell them, "Hey, you really didn't tell me why your character thought death was a Good thing. Maybe you should think about being Lawful Neutral, or come up with a better way to explain your characters logic."
Try not to limit your Players creativity. Let them have some head space! I have a Halfling barbarian, who is quite crazy. Is he a good character? Meahhh, so-so. Is he fun to play? OHHHHH yeah. Try and think of how and why the PC's Character COULD exist, not what the Players Handbook says is the norm. Of course, if in your world it is physically impossible for a halfling to rage, thats your thing. But maybe this one is different. The Chosen One of the Halfling tribes. Let the PC's have a little part of creating the world they play in. Let them be different or set apart from the normal adventurer.