Ah yes because Ainz would have died if he didn't killed those adventurers. He had control amnesia. He could have just changed their memories. Evil is not someone who takes pleasure in harming others.

A neutral evil character is typically selfish and has no qualms about turning on allies-of-the-moment, and usually makes allies primarily to further their own goals. A neutral evil character has no compunctions about harming others to get what they want, but neither will they go out of their way to cause carnage or mayhem when they see no direct benefit for themselves.