I meant more along the lines of what Universal would even consider and what could avoid a nasty fan backlash.
What seems better is for Chomper to hunt Sharpteeth (look, that DID happen in real life), but have him framed for killing a Leaf Eater, have a falling out between him and Littlefoot, have him truly almost turn on Leaf Eaters, only to find out that he'd been set up, get the real perp, and the two would stay friends.
Also, the argunent that he CAN'T eat other Sharpteeth is silly. Yes, they have claws and stuff, but the Leaf Eaters have things like long tails with bull-whip attacks (Longneck), Spikes (Spiketails), horns (Threehorns, which, as you know, are the T-Rex's main rivals in the Leaf Eater world0, clubs (Club Tails).
Also, as you know, it's not just Chomper who might get PTSD. It would also scar Littlefoot.
He lost his mohter to a Sharptooth. If Chomper goes the route you suggest, he's going to wreck OTHER dino's mothers.
You think that's NOT gonna occur to Littlefoot? Who said Littlefoot would let Chomper live when that occurred to him? Heck, who says the GV might not catch on to the direction Chomper is heading, and kill him before he can strike down the first Leaf Eater? They were willing to kill him in "Brave Longneck Scheme".
Plus, we already have a situation in LBT, where, if you think about it, many characters shoulda been wiped out by now but aren't: the Yellow Bellies.
If they can not get wiped out due to natural selection, we have some hope that Chomper will find a way to stay off eating Leaf Eaters.
LBT has lowered the bar of believability with the Yellow Bellies and the aliens in movie VII already.
(PS, maybe Chomper will eat Yellow Bellies. That might solve some problems!

)
Besides, I think LBT would have the GV be infested by evil Leaf Eaters that kept coming back for yeras and that Chomper had to keep eating to keep the GV safe rather than actually having one of the hero characters like him ever truly being portrayed as a villain.
I'm not saying that I don't believe that a Chomper eating Leaf Eaters scenarios could work in a fanfic.
I just said that I cannot see it happening without him having some sinister fallout to his character. (The Chomper we know WOULD die before he killed a Leaf Eater.)
I can see a scenario where the Great Valley, minus Littelfoot & Co. treats Chomper like an evil Sharptooth, and eventually they expel him.
Chomper, who WAS nice, gets bitter inside and indeed does turn on Leaf Eaters, having all ties, minus Littlefoot & Co, whom he trains himself to forget about, severed with Leaf Eaters.
In time, he joins with another Sharptooth, who would be the real villain of the plot, and said Sharptooth tricks Chomper into the agreeing to help him attack the Great Valley.
At some point, Chomper and Littlefoot would get into a fight, but Littlefoot would get Chomper to agree not to attack him.
Eventually, the true villain Sharptooth would be defeated. Chomper would be too tainted to come back, but would agree NEVER to harm anyone in the Great Valley and Littlefoot & Co would visit him from time to time.
Tragic, but doable.
If Chomper is a deceived character, it CAN work, er, maybe. If he actually IS the villain of the plot, it's gonna lead to trouble and that's a fact Jack!