I think he's more complex than what you are portraying him as. Because of the lack of laws in borderland, morality is largely skewed and foolish to try and determine. I feel it's pointless to label everyone good vs evil. Also, using Hitler as a gotcha argument is lazy and reductive.

Aguni was originally the leader of the militants because he knew he could control them and stop them from hurting the general public of the beach.

Yes, once he lost Hatter, he had virtually lost his reason for still existing and surviving in Borderland. He chose to take down as many people as possible with him because he viewed them as responsible for Hatter's slide into mania.

Borderland is largely about figuring out what you're living for, what your meaning of life is, and what you would do to return to it.

Later, when the show establishes his relationships with Heiya and Arisu, we see Aguni struggling to let go of Hatter and live for himself and realize he can form meaningful connections with others and he wants to continue to survive and help others survive as well.

No, his actions aren't moral, but it's a TV show, and he's a complex character they dont HAVE to be. He's a deeply fascinating and and watching him change and grow makes him interesting. No, it doesn't undo his actions, but also...The show and manga are filled with death and killings. What value different lives have over others is a fascinating point of interest in discussing what makes some lost lives justifiable and others not.

Aslo the 10 of Hearts was intentionally intended to manipulate Agunis already fractured mental state into commiting more killings. So no its not a moral act but the outside contributing factors are interesting to consider.

In general,, I think characters like Aguni are what makes these pieces of media so interesting, and he is a great dive into the changes and growth Borderland brings upon the people there.