The shalya thing was completely because of their own foolishness and penchant for trickery. Seriously, why would you trick someone and his large army to fight for you when he wants to be part of the opposition? good for them shalya didn instruct his army to kill kaurav soldiers instead of harming the opposition -- someone else in his position today would not be forgiving and would be fueled by vindictive feelings to do who knows what? And then you go and make him a charioteer when he clearly objects to it ...