*just copying and pasting from a convo I had with Owls on another thread cuz I'm too lazy to write an original response*

In my opinion, I would still argue that Sharptooth was evil...or at least the most evil in comparison to the other antagonists featured in the LBT movies. It feels like such a waste of energy to chase the Gang of Five for who knows how long, and still end up empty handed. I guess we could also assume from his perilous pursuit that he was unsuccessful in capturing any other prey off screen, so Sharptooth has to resort to getting a meal that is easy to get (seemingly) but at a huge drawback of any nutrition. Being in an environment where leaf eaters are pretty scarce due to the lack of vegetation also doesn't help the situation at all. But then again, what about all the herds traveling to the Great Valley? There were a lot of casualties, so theoretically, there should be scattered corpses all around for a Sharptooth feast, correct? Was that ever shown in the movie? We also have to factor in the Diplodocus herd that ran up to the trees and ate up all the tree stars that the Gang should've had. Where the hell did they went after that? I have no clue honestly. The last factor to consider is the chance of an encounter. With all of these food jackpots littered across the land due to the Earthshake and the vast expansiveness of the Mysterious Beyond itself, what were the chances that the Gang would encounter Sharptooth not just once, but twice!? Unfortunately, a decent chance because both the Gang and Sharptooth were traveling towards the direction of where the fabled Great Valley supposedly exists. But even then, if Sharptooth had some sort of consciousness and a bit of empathy, he would leave the kids alone and move towards the food source that is plentiful AND easy pickings at the same time.
So with all things considered, I now think that Sharptooth could've made much better and more rational decisions in the first movie, but he isn't a complete monster. Very flawed, but maybe he thought he had no other choice but to hunt the children down, otherwise face starvation.
Poor Littlefoot, what an unfortunate situation to end up in. I kinda wonder what thoughts were running inside the Sharptooth's mind the moment he was hit in the eye with the thorn - albeit by accident because Littlefoot was trying to escape from his pursuer. A normal and rational mind would shake it off and let it go, but not for this guy. I would even say that he wasn't completely open minded before his eye ends up getting injured, making him more enraged. During a hunt, the natural survival instincts probably held mostly dominant over his normal cognitive abilities. So when the Littlefoot accidentally hurt him in the eye, it was most likely taken as some sort of challenge by Sharptooth, and therefore that's how the endless chase for the blood and meat of the Gang started.
Or we could just say that Sharptooth is a very egotistical guy in general and doesn't like his pride to be hurt like that. The more simple explanation of course, but at the same time, not explaining the full scope of possibilities though.

And the really, really simplified explanation is revenge.

For a little addendum to my original two posts, I feel like the version of Sharptooth depicted in the book was simply made just to appeal to the masses of people who just want to confirm their bias that he's all
and only evil. While I also hold the same bias to a certain degree, I agree with viewpoints from
both the book and what the intended message that Don Bluth was trying to push out to the viewers.