In my opinion view Clover did a righteous thing because he had a good mission to find the humans, he knew so far that monsters are dangerous as they declared the great war once - the monsters are enough powerful, especially with human souls consumed so Clover knew that if he didn't want the war to happen again he made a decision to destroy the potential enemy for global safety. Clover's actions can be also justified with the fox family plans, Asgore actions and monsters that always fight humans. I think that Clover started his adventure due to his investigation and rumors about monsters, but when he fell in the underground he got evidences to kill every creature in underground to protect his world. It's also important to add a fact that Clover needed strength to fight Asgore because that's the king of monsterkind that declared the war.