It's simply because the game mixes pixel based collision detection with physics, so sometime only one out of two different collision types will happen, in which case a character might turn but not kill the player. Physics are a bit quirky, at least the old version of Chipmunk where it's hard to detect if two objects are touching each other (I could only really test "on collision" easily). Anyway, this will unfortunately not be fixed, I worked hard enough to get it to work as nicely as it actually does in the game
