Pettigrew did show the moment of mercy to Harry. As I've said before, he was a coward and it's pretty unforgivable what he did, yet at the same time, I feel he was not quite at the level of evil of Bellatrix or Riddle. He didn't seem to take joy in the position he was in, as a servant to Voldemort, and ended up where he was because he was too cowardly to resist Voldemort.