I really hate Yuki.. seriously, theres NOTHING good about him, he is retarded, thinks that killing is wrong even though he is scared of dying himself, and is willing to "injure but not kill" others, which is retarded, because he always ends up killing them. That part about him killing others is NEVER crossing his mind, and he only thinks that other people kill, not him.
Marco and Ai was probably the best canidate for willing, seeing how selfless they are.
Yuno isn't bad either, she is just insane and like Yuki for some weird unsupported reason. She is always the one fighting, and cares about Yuki only, and is willing to sacrifice herself/others for him.
This is manga is actually more like Yuki is bad, and Yuno is KINDA bad... and everyone else are mostly good.