Smart. And sneaky. I like it. Alot. But...
For more straightforward types though, incapacitate him.Tie him up. Use a spell to make him tell the truth (zone of truth, discern lies etc.) or Intimidation. Kill him afterwards (using coup de grace if you like), and throw him in a shallow grave.
But charming a wizard (high will saves) is hard, plus his has spellcraft so he knows what you are up to. And if you think you've got better DC than he has will save, then probably you've already got better spells than him...so kill him just for xp.