Rino was reflecting. He did not mean to get involved directly. However, when those trolls came a little too quickly at sunrise to collect their dead comrades, Rino was still in the middle of waiting for the last troll to die.
He couldn't let such a good body count go to waste, so Rino made his appearance and tried to negotiate with the trolls. However, he forgot that they were monsters with little intelligence. Reasoning with them was a failure from the start.
Most of them were starved and lacked any real threat. However, Rino was in such a desperate situation with the labour crunch he was facing that he didn't think. He only reacted and killed those who attacked, which happened to be everyone on site.
Reader Side quest:
Comment if you like Rina or Zerg better.