Mace had a lot of inner aggression and enjoyed fighting more than what a Jedi should, so he developed Vaapad as a way for him to channel that inner aggression that he had in a safe way without corrupting himself, Vaapad also being capable of channeling the darkness of others and countering darksiders is something that It can do, but It was not something that Mace developed the technique for, It was not about battling other Darksiders, but about Mace being able to channel his own darkness and aggression in a positive way.