Of course such casting is an evil act. The books specifically say so.
However, there is an escape clause. The books also state the evil descriptor can be removed or ignored, and the spells made a part of the general spell repetoire. In that case, casting the spells is not an evil act.
It's up to the DM to decide this matter, as usual.
Personally, I think the game is more fun if the vast majority of spells (including most with the evil descriptor) are in the general spell pool. But it is nastily entertaining if certain gruesome spells are restricted to the evil guys: that way, the party will know it's facing some serious stuff when they attack the evil cleric or evil wizard they've been hunting down.
For example, let us say there is an Evil spell that requires the sacrifice of a virgin elven girl, and grants an Autokill of a most messy nature in return. Now, unless it's an entirely evil party, I doubt any players are going to want to take this spell due to it's material component requirement. But if the NPC the party is after has this spell readied, the party knows they face a real problem if this guy gets the jump on them!