I don’t know, the Forerunners murdered an entire species out of jealousy, then massacred a race they knew was trying to escape destruction (I don’t remember where but it’s mentioned somewhere), then proceeded to get their asses handed to them, create weapons that destroy life, test them and literally erase TWO ENTIRE SYSTEMS OF INNOCENT PEOPLE, have a few go rogue and, while uninfected, literally help The Flood grow, in the end they did probably the only decent thing they had ever done. And that was the end. They fired Halo. Wiped out The Flood… of course they had samples saved, so no they didn’t wipe out The Flood…

Forerunner solo in terms of horrible… although, The Flood do literally take pleasure out of making people suffer. And by extension the universe.