Stupidity: When Trystane is engaging Nymeria in combat, he pivots right in front of Obara, whom he knows is armed with a spear, and leaves his entire backside exposed. Anyone with a modicum of sword training would know not to expose themselves to an armed opponent like that. He ends up immediately being speared through the back of the head because of it.
Suggested correction: They ask him who he chooses to fight. He chooses Nymeria, believing the others will not interfere. He was wrong.
Two women snuck aboard his ship intending to murder him, and he trusted them to be honest about fighting fair? That just adds to the stupidity.
That's naivety at best, not stupidity. Plus they didn't sneak on board, they announced their intentions.
If two armed people entered your house and told you they were going to murder you, but would give you the opportunity to fight your way out, would you actually believe them as well as immediately turn your back to one of them? Trystane is a prince who undoubtedly had education and combat training. This goes beyond naivety.
You are forgetting he is also only 15 or 16 years old and never left Dorne (what I take from the show). What does he really know? He probably never actually fought anyone in his life.
The women that killed him were previously locked in a cell for trying to kidnap his betrothed, which he knew about. He also knew she died on the journey back to King's Landing under suspicious circumstances, and likely knew it was from poisoning and that they were involved. Based on the show's lore, someone in his position would have had schooling about the world and its people from a maester and combat training from a master-at-arms. He knew full well not to trust these women.
