Still, the saber-toothed cat Diego rises above his own urges to eviscerate the baby and eat the mammoth because he learns the power of true friendship and responsibility in their company. Together, they fight off the other saber-toothed cats, and then the ragtag group of plucky yet utterly doomed species wanders the barren wasteland together until the next big storm.
The Ultimate Betrayal
In some of the soundest logic in the entire movie, Diego and the smilodons hatch a plan to lure the mammoth into a deadly trap where they all can kill him. Now, just to be clear, killing other animals is the only means that predators have to eat. They aren't just murdering a furry elephant because they're curious what sound it will make while bleeding out in the snow. They are relying on that meal to avoid starvation.
Or rather, they would be, except Diego betrays his pack, as well as thousands of years of instinct, by becoming friends with the mammoth instead. What's more, Diego tries to kill the other saber-toothed cats when they're just following the plan he orchestrated.
To the mammoth, that might seem like a heroic move, but from the viewpoint of Diego's starving pack, that's the shittiest thing he could have done to his friends. He even contributes to the death of the pack leader, causing Soto to be impaled by icicles.
The most prescient thing Sid does all movie.
Now short two of their best hunters, there's a good chance the other smilodons won't last much longer in the harsh conditions. Diego himself won't be able to hunt alone, and certainly not with a mammoth and a talking sloth lumbering behind him. He's just ensured the collapse of one of the last functioning animal societies in the ice age.