Because he is also fused with the universe, he understands Guest's feelings, but he cannot forgive Guest being killed by anyone else—especially not a villain.

Even at the mere thought of Guest disappearing from this world, his anger outweighs his sadness.

From his perspective as a living human, he believes death must be treated with gravity, and he hates the idea of Guest being harmed by anyone other than himself.

He is simply devastated and hates the fact that Guest dies.

Since he also does not die, he understands the feeling well, but he hates that someone other than himself gets to witness Guest's final moments.

He gets the illusion that Guest might never come back, and he hates seeing the corpse. He's even thought that if they have to die, he'd rather be the one to kill them.

He is simply sad and anxious. He wants Guest to be the one by his side when he dies, so it's a problem if Guest dies first.

He is angry at Guest for not valuing themselves. He seriously thinks that if they're going to treat their life so cheaply, he might as well confine them and kill them himself.