These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
When the Hulk first challenged him to battle, the Red King casually kills off the rest of a gladiatorial bout's survivors (save for Miek); and, after Hulk gains the upper hand, he sends his guards on him and shoots him in the back.
Later, we see him firsthand casually ordering the death of people who say something against his high tributes and dismissing the complains of his peasants, because he doesn't see them as anything more than animals.
During the second Arena Fight of Hulk and his Warbound, he screws the rules and tries to nuke them (together with the whole audience), which only fails because Hulk contains the blast. He even attempts to make them kill one of their Warbound-members via Mind Rape.
After the heroes confront him with his crimes, Miek using chemming to force him to relive them, he doesn't even care one bit, rolls his eyes, and smugly proclaims: "Yes. I killed them. As is my Right. As is my Duty. As is my Pleasure."
Plus, Caeira's flashback of him as a child. In the comic, he saves her life but immediately enslaves her. In the animated film, she offers her loyalty willingly but the kid Red King turns out to have been still just as villainous when he reveals he ordered the Spike attack on Caiera's village specifically so he could capture her.