Killing your own clone is still murder.
I've never really liked Riker - either of them, so I can understand why they don't like each other. But if I met a person who had spent years alone on a planet trying to survive, even my worst enemy, I would never treat him the way Will treated Thomas. Or the way Picard or Worf treated Thomas. Even a total asshat deserves some compassion after going through an ordeal like that.