Is it bad form to attack another player who antagonizes you and insists you are not teammates? /u/shadowsog95 DnD: Roll for Initiative!

I played a game with my brothers a while back and the scenario was we were all being human trafficked by a band of pirates and we fought through the ship and got to an island with a cave full of skeleton soldiers and I picked up a sword that started the encounter (I’d been a weaponless tabaxi barbarian up to that point) and one of my brothers kept being kind of an ass as a holding rouge just messing with everyone’s actions on purpose with the excuse of “we’re not a team yet and I don’t know you guys” but at the end of the encounter all the enemies and weapons besides my sword disintegrated and the first thing he did was “pickpocket” the sword that was in my hand that I was currently using then run down a dark pathway in the cave with no turns or exits. And my response was to chase after him and beat him up for the sword and everyone else thought I was taking it too far but 1. He shouldn’t be able to pickpocket a weapon I was actively using, 2. He shouldn’t be able to disappear into the darkness when I have night vision as a race ability. And 3. He’s being an antagonist to my character who would 100% kill a random theif they didn’t know for stealing from them according to their backstory. At this point we were all level 2~3 and he metagame joked about how his rouge could kill my barbarian easily.

I played a game with my brothers a while back and the scenario was we were all being human trafficked by a band of pirates and we fought through the ship and got to an island with a cave full of skeleton soldiers and I picked up a sword that started the encounter (I’d been a weaponless tabaxi barbarian up to that point) and one of my brothers kept being kind of an ass as a holding rouge just messing with everyone’s actions on purpose with the excuse of “we’re not a team yet and I don’t know you guys” but at the end of the encounter all the enemies and weapons besides my sword disintegrated and the first thing he did was “pickpocket” the sword that was in my hand that I was currently using then run down a dark pathway in the cave with no turns or exits. And my response was to chase after him and beat him up for the sword and everyone else thought I was taking it too far but 1. He shouldn’t be able to pickpocket a weapon I was actively using, 2. He shouldn’t be able to disappear into the darkness when I have night vision as a race ability. And 3. He’s being an antagonist to my character who would 100% kill a random theif they didn’t know for stealing from them according to their backstory. At this point we were all level 2~3 and he metagame joked about how his rouge could kill my barbarian easily.