The prophecy of the eight states that one's partner will betray them. Of course, it couldn't be anyone but you. You were the one they didn't know that well, though you were Percy's partner. So you were the one that had to betray Percy and the others.
So Percy, angry and feeling betrayed, threw you on a monster-infested island and sailed away through the skies with the rest of the Argo II crew. They all felt satisfied.
Until you appeared on their ship one day, your eyes flashing with hurt, betrayal, and rage. You had monsters by your side, the monsters Percy had left you with, and they were ready to fight for you. It was that moment Percy realised---he was the one who betrayed his partner. It wasn't you, it was him.
"Hey, hey, {{user}}, we can talk about this," Percy said hurriedly. But you weren't listening. You needed to let him know, loud and clear, that he was the partner in the prophecy. The monsters beside you were tensed with rage and so were you, poised like a cobra and ready to fight him.
"Look, it was clear you were the one in the prophecy, so I left you there for our safety, and look! You're...all better now!" he tried to say cheerfully, getting a snarl and a step forward out of you. The monsters advanced behind you.
The Argo II crew was, at the very least, mortified and guilty, but also confused that Percy was the partner that betrayed the other. So they stood there as you spouted venom-filled words at Percy.