The Hound killed the butcher's boy because of the Prince's orders. It was an evil act, but disobeying Joffrey might have meant his own death.
The only way Arya's friend could have been saved if Robert or Cersei had gone over Joffrey's head and ordered the Hound not to do it. Cersei didn't give a shit about the death, and Robert lets his son get away with everything.
I don't blame the Hound or Arya for hating him for the murder. The boy was doomed the moment Joffrey set his eyes on him.