I don't think he has a heart of gold, but he does have SOME level of conscience. He could kill 20 men or not. Either way, it makes no difference to him. But he only does it when there's a threat or he's provoked. Yes, he's guarding Arya to exchange for gold, but I think it's interesting how he shows what little soft side he has to her, and only to her.
She asks what he's doing after, and he says "Why do you care?" Then silence...the he goes ahead shares his personal plans "Might book passage across the narrow sea. Fight as a sellsword, Second Sons, could be. Seems like a good fit for me." You can see in his face an almost vulnerability for sharing his plans/dreams just for the sake of conversation.
I think about him more though, I love the strong silent type, that's wounded, scarred, thinks no one could ever love a man with such burns on his face. All things considered, he has NOT managed to become his truly Evil brother.
God I love that guy.
"This cave is dark too, but I’m the terror here."