Red Skull is much more cruel than Cell. I don't even recall Cell enjoying intense levels of messed up torture that borders on insanity, absolutely inhumane. And I do not agree about the "he only wants to kill some people", humans or mutants, he's going to slaughter anyone of any ages of any origins.
As much weird it sounds, Red Skull is literally worse than Hitler in the comics and beyond messed up even for Nazis' standards, I even doubt he even follows the ideology at this point and goes full ham at times. I don't think I can mention his actions due to how messed up they are (don't want to get blocked lol)