You may be a Gentle Sharptooth...but Littlefoot is a very rough leaf eater. 
Indeed, Littlefoot and how he handles green food makes us sharpteeth seem civilized when when we chow down. 
Argubly, even more humane than Littlefoot. Just look at poor Spike, grunting in pain and suffering in his final moments on Earth as the heartless longneck eventually kills him and cooks Spike on a barbecue.
That’s why Littlefoot raised Chomper. He’s part of the Secret Sharptooth Society. Why do you think he acts so excited about treestars? Its a cover. Spike doesn’t stand a chance.
I wouldn't be surprised if Littlefoot actually faked Sharptooth's death. It was all a ploy, a trick. Once the timing is right, they'll both kill everyone and rule the Great Valley.
As for Sharptooth trying to "kill" Littlefoot, they had to make the scene believable so nobody would suspect a thing.
Exactly, and what better way than for Sharptooth to kill Littlefoos mother. The Gang then would never suspect Littlefoot was in league with Sharptooth. This also explains why Littlefoot chose drowning Sharptooth over lava, this allowed Sharptooth to pretend he drowned, sink to the bottom, and then when the Gang left, return to the surface.
