Oh my. For large N ( > 400) this gets an exponential structure of strings of carnage that can be stopped at certain points: a simple strategy exists for each pirate of fierceness 400+100*2^n (negative n allowed until we start getting fractional pirates). No optimal strategy exists for the remaining leaders; they die.
Let's start with the first carnage at N>800.
There's at most 200 pirates who got any gold, and would prefer to keep it, but they have no say in it, for there's another 600 blood-thirsty jerks who believe they can murder the fiercest with full impunity and have nothing to lose (and either gain a chance for gold, or not gain anything).
The leaders are slaughtered until the total number drops to 800. Then the leader distributes gold to the bottom 600 at random (1 piece per pirate though).
Each of those on the bottom, who got a gold coin will vote to keep it, because they won't earn more anyway, and have a chance to lose it. Then the top 200 including the tie-breaking boss joins with approval and with 400+tie votes they outvote these out of bottom 600 who didn't get any gold.
Now, assume one of the top 200 is an idiot and votes for sharks at the critical point. 200 satisfied with their coins and 199 currently-victims won't outvote the 400 who want blood (and money). The 400 keep killing until there's only 600 pirates left: 300 without gold, 200 with gold, and 100 potential victims if they don't come to agreement right now: gold distributed at random to 200 out of the bottom 500, the 100 come out alive, 300:300.
Again, assume an idiot botches the voting. 250 blood-thirsty pirates slaughter 50 leaders, then best 50 of them join the 200 satisfied to vote own survival.
You see the pattern?
Let's try upwards. In our 800-pirate population we have 600 blood-thirsty jerks, who need to be outvoted by 200 with gold and a certain number who just wants to live. Well, that number appears to be 400, bringing the total number to 1200. But if the population is bigger, these 400 know they can kill with impunity (and won't get any gold anyway) so 1200 is the next population threshold.
So, 25, 50, 100, 200, 400...
500 = 400+100, 600=400+200, 800=400+400, 1200=400+800
Each pirate of fierceness 400+100*2^n has the opportunity to come out of this penniless, alive, and stop the slaughter, in exchange for distributing all gold to the bottom 3/4 of the band, randomly.
Each other pirate above these specific ones, I think 400, correct me if I'm wrong, gets fed to the shark no matter what they offer.