I just want to clarify something here: During the earthquake, when Sharptooth falls into that huge cassavas, how did he survive that fall? I have watched a few documentaries that state that a fully grown T. Rex would die from its injuries by simply tripping over, let alone plummeting free-falls. Also that after Cera wakes Sharptooth up, all that he got was apparently a sore back!
So in that case, Littlefoot's assumption that he was dead was perfectly justifiable and that he didn't need to go to the extent of finishing him off at the lake.