Perhaps if Neil was threatened by an adult? Lex hates adults, and he wouldn't hesitate to kill a lone adult, especially if he thinks he can get away with it. Now, I'm sure Neil would be more than capable of defeating the enemy, but I believe you mention his reluctance to fight. While it would be somewhat out-of-character for Lex to challenge someone head on, he would definitly sneak attack. Maybe Neil could try to escape, but gets cornered somehow, and Lex swoops in and kills the adult. How does that sound?