Gus ordering the murder of that kid (who had killed Combo) and murdering Victor in front of Mike, Walt and Jesse, are both far worse than anything Walt ever did. And remember that Gus used to routinely go taunt an old man in a wheelchair who could no longer speak. Yes, Hector had murdered Gus' buddy 20 years earlier, but still, that is a pretty monstrous act.
That aside, he was a fantastically written character, and Esposito did a phenomenal job playing him.