When you are faced with two or more monsters, can you kill one and flee the other?

No. If you have cards that let you abolish one entirely (like Pollymorph Potion) then you may do that, and fight the other one(s). (You will have to win that fight before you can claim any treasure at all . . . you can't charm one monster, grab its treasure, and then try to fight its mate.) But you can't fight one and flee the other. They fight you together.