Have it start with using evil spells for a just cause after taking a dead necromancer's spellbook.

First with useful combat spells when they would be useful, like enervation or wrack.

Then with animating corpses and other evil acts but only when necessary: The party paladin dies and no one can lift the corpse with all his armour on? Animate him, and make him walk on his own. The group's horse dies on their way somewhere? Animate it. They can't carry all their stuff on there own, can they? Recurring villain? Hit them with contagion.

Eventually his character should start sinking into actual with unnecessary killing. It could start with partially justifiable things, like the orc's women and children, or killing and soul binding the recurring villains, and stuff like that. Then it would go to killing the bystanders, shooting the messengers, murdering those kids that were playing ball next to the inn noisily and stopping him from sleeping and refreshing his spell slots, the like.

Make sure he roleplays his intelligence: even when he becomes completely evil he has no reason to start kicking puppies (Unless the puppy has a golden collar or something). He might even spare some of the PC's (after turning on them while they're weakened) because of their former friendship.