Why was Captain Flint's body there with the treasure? There weren't any other bodies around, and it didn't looked like he'd been killed or anything. Also, what happened to the crew of his ship? They surely knew where the treasure was, since they were the ones who hauled it there. So why didn't they come to the planet after Flint's death and get the treasure? (Or, if Flint had already switched the security system on, why hadn't they accidentally set it off and blown up the planet while trying to get the treasure?) Okay, you needed the navigation ball to get inside the treasure trove, but clearly one, or some, of Flint's crew had gotten hold of the ball, since it ended up outside the planet... So why hadn't the crew member(s) used the ball to get the treasure? Or, even if the they didn't have the ball, they should've at least tried to get to the core of the planet with some other means. With the technology level shown in the movie, it shouldn't have been impossible.
Multiple (and possibly inter-connected) theories:
On the crew: A) Maybe most/all of his crew died along the trip due natural hazards, and Flint's ship was for some reason too damaged to escape, so he starved to death in that cave B)Or Flint is just that jealous with his treasure and stood there and starved to death on purpose because of how damn protective with his treasure he was ("No one will have my treasure, no one!"). On the Ball: A) Maybe some did survive, but were too scared of Cap'n' Flint and his memory to even try, and so they purposefully buried everything they knew (including the ball). B) Maybe B.E.N (who was a member of his crew) was the only survivor of the trip, and left with the ball (not knowing what it was for since his memory was ripped off), after a series of unrelated events the ball got lost (one also needs to note you need to know how to use the ball for it to work, see Silver messing with it to no avail in one scene).
It's perfectly possible that Flint killed his entire crew to keep the treasure all for himself.
Why did the pirates plan to kill the other shipmates. Unlike in the original novel, which was like one treasure chest, which is alot of money, but can be stretched thin with a large group of people. This is a WHOLE PLANET of treasure. One treasure chest from that place would set you for life, in the end of the film, a hand full of diamonds rebuilt the inn (Which is maybe like what 100,000 dollars). The pirates couldn't share a planet full of treasure with like four other people? If the pirates hadn't backstabbed the other crew members, they would have found the treasure with no hassle, collected as much diamonds and gold as they could, become incredibly wealthy, and gone on there way.
Wild Mass Guessing on my part, but since the Legacy was captained by Amelia who was in full uniform, it's believable that the expedition was being authorized by the government. Once they found the treasure and got back to Montressa, the crew would only have gotten their standard pay, maybe a little more. The rest of it would have gone to museums, or simply taken by the government itself, depending on the current morality they exhibit.
Well, what's the exchange rate on that planet full of treasure? It could amount to chocolate coins in tinfoil for all we know...
Greed, simple greed. Four more people to share the treasure with. Also they didn't know for sure there will be mountains of gold. According to the legend there are tones of treasure, but they can't be sure. There could be as well a few chests.