He just had to turn on me and try to warn the guards about my arrival on Kingsparrow Island. I proceeded to put a bolt in his head and let the fish eat him. I really liked him, too.
I did the same thing on my high chaos playthrough. Actually, I reloaded to try and catch him with my sword once, but I ended up missing and having to blink/swim chase him down until he drove off the map, to which I reloaded again and did as you did.
On my first high chaos play through he didn't do it. Second time I knew I was aiming for the highest chaos and he did. Was so confused that I laughed in spite >.>