The only argument I could see is that Walt did a dastardly act - poisoning Brock - against an innocent person, while all of Gus' monstrous acts were committed against people who were bad people, too (the kid who shot Combo, Victor, Hector, etc.). But even still, everything Walt did, he did out of self-preservation (for himself or Jesse, his partner), while Gus taunting Hector served no other purpose other than to add to the misery of an old man in a wheelchair. And Walt did seem genuinely remorseful about poisoning Brock (see: his reaction when Jesse tells him on the roof that Brock was touch and go for a while, but is gonna make it); Gus was never remorseful about anything he did.