From what it sounds like. It's not much of a shock that the Fishmen/Mermaids kill each other. It'd be weird to have a lawless district where it didn't happen occassionally at the very least.
I think I didn't express myself properly - it's certainly not odd for fishmen or mermen kill each other. Otohime, on the other hand, is a symbol. She appears to stand for understanding, not just the humans, but those of her own people who need it. She is not merely a human apologist. Her people may be uncomfortable with her desire to get to know humans, but she is beloved among them for who she is - a queen who really, truly cares.
The Sun Pirates, I suspect - Jinbe in particular - treat her as a naive idealist (which is certainly true at least in part), but killing her is on a completely different level from sneering at her. Killing the queen who loves your people - and no matter what else, they know she does - is not the same as killing humans whom you hate because they enslave your people. A line was crossed.
Otohime probably let herself get killed for whatever reason.
Interesting you should say that. Returning to the analogy with Mahatma Gandhi, he faced four (I think) attempts on his life, and warning had been received of the stalkers who ultimately succeeded in killing him, but he refused security - he always believed he would die a violent death. Otohime's clearly ready to injure herself severely for a thief; it's not unlikely that she may have been willing to martyr herself for her cause. Her death certainly had a strong effect on Jinbe.
Maybe Otohime simply refused to fight back against her killers (possibly bitchslaps proved ineffective at getting them to reform - or they dodged). She probably forgave them with her dying breath, too. And smiled while she died.