Question why he wants to go to lunch with you(most likely ask yourself, maybe him if you're feeling brave.) If he hates his brother so much as you said, and he's asking the person his brother just asked to have lunch with, is he doing it because he likes you, or to spite his brother? Do you really want to go have lunch with someone who's brother seems that unstable, to pull a knife on you? Who's to say the knife guy won't get jealous and attempt to hurt his brother or you again, this time without warning?
Overall, this just seems like a very dangerous situation. If you must approach it, do so with caution.