Rorschach being good is debatable, he’s a Batman like vigilante who hasn’t the ‘no killing’ rule, which is dubious.

But the reason he chose death was (in my humble opinion) that he realized Veidt had found the solution, that would bring peace and create a world he would be useless in.

This point is made by the ultra nationalist frontiersman publication he sent his diary to. They complain that they have nothing to write about as the world was united in boring peace, this is when the burger munching intern gets the assignment to pull something out of the loonie pile.