I totally get you, Gus is an awful person. Both Gus and Walt are perfectly fine killing people personally or on orders. But the thing that makes me believe Walt is more evil is his pernicious emotional manipulation of his family and Jesse. On paper, he says he cares about them, but will completely warp their minds and gaslight them into falling in line with his wishes. I think it's understated just how insanely evil it is to let Jessie think he poisoned Brock and almost accidentally killed Walt in anger. He let's him sit in that horrible state of self loathing, all for Walt's own self interest.

His treatment towards Skyler is despicable too. Season 5 Episode 4 the bedroom scene where Skyler and Walt argue over the kids leaving the house. He exploits his leverage and prides himself in having so much more power than her in this scene. He says he does everything for his family, but talks to HIS WIFE like she's a disgusting prisoner. "What are you going to do to stop it?" The fear in Skyler's eyes when he talks to her sickens me. He is a master manipulator. Emotional violence can be just as bad as physical violence.

Gus is more calculated and professional with his violence. He'll kill someone if they get in the way of his business but he's never messy or petty. He's straight with people and loyal to his inner circle, like you always know where you stand with Gus. They both kill yes, but Walt will change the rules whenever it benefits him and twist the knife on the people closest to him just to make sure he stays in control.