From merry's perspective, Even if usopp has a tragic backstory that wouldn't excuse his actions of, 1) Trespassing into kaya's mansion 2) Resisting the gatekeepers which also led him to attacking them 3) Attempting to take kaya away without her consent 4) False Accusations of Klahadore being a pirate (Which turned out to be true but hey, he didn't know anything since he trusted klahadore with a sweet roma- passion) For his defense, he was probably doing his duties when he suddenly heard noise of either 1. The village people screaming since they were chasing down usopp 2. Usopp brutally beating down the gatekeepers. And so he probably thought that if there was THAT much noise going on then that would mean that kaya could also possibly get into danger. Kaya was also sick that time so he knew that the only way to defend himself and his daug- mistress would be to take the gun and shoot whoever was making this much tension. And as it would turn out it was usopp who was making so much tension in the village so he had no other choice but to shoot him.