Although he is the main villain, his role in the film is small. Due to his immense strength, he was the leader of his pack, leading his pack in hunts for food. After several of his fellow pack members were murdered by the humans, he was taken over by the temptations of revenge, acting out on a perverted vision of justice, choosing to go after a tribe of humans instead of migrating down south like all the other animals.
Executing The Plan
Soto declared that they would kill off the humans' youngest, a baby named Roshan, as an act of revenge to strike back at the humans. Soto and his most trusted pack member, Diego, plan an attack on the human camp, so that while he and three other sabers would attack the humans, Diego would slip in, stealthily, and take Roshan without being noticed, as a chance to put their plans into motion. Soto then ordered Diego to alert the others for an attack at dawn.
The very next morning, Soto and his pack ambushed the camp, taking attention away from the tent in which Roshan was located in, and as planned, Diego takes advantage of this and slips in, undetected, to take the baby, until the baby's mother, Nadia, hurried in and snatched the baby before Diego could do so. The ambush failed and the pack was forced to flee from the camp. Diego would, later, return with the baby, and Soto thus gave Diego the order to meet them at Half Peak, with the baby.
On the way to Half Peak, the pack stopped for the night and Soto sent out two from the pack, Oscar and Zeke, to deliver a message to Diego, who was accompanied by Manny and Sid. Soto told his two pack members to deliver a message to Diego, informing him to either come back with the baby or to not come back at all. Diego, in response, told them to inform Soto that he'd bring back the baby, along with a mammoth.
After getting closer to Half Peak with the mammoth, sloth and baby, Diego once again rejoined with his pack and met with Soto, who was beginning to get anxious about Diego, but expressed his approval at Diego's efforts. Unbeknownst to Soto, however, Diego had joined forces with the mammoth and sloth, in his own plan to save the baby and escape unharmed.
The pack lay in hiding at Soto's orders, who were ordered not to move until they saw the mammoth. Diego, who was trying to sabotage the attack, convinced Zeke to jump out of hiding too early which caught Sid's attention. Sid, however, knew it would happen and sped away with a bundle in his paws on a pair of skis fashioned from two large pieces of bark, to be chased by Soto and the others until Sid lost his footing and dropped the bundle of clothing, which Soto pawed at, only to find it was a decoy made of snow. An angry Soto ordered his pack to get Sid. Soto then found Diego and Manny and invited Diego to help bring the mammoth down, only to find out about Diego's betrayal. Enraged, Soto fought with Diego while Lenny and Oscar cornered the mammoth. The fight between the two was evenly matched until Soto threw Diego into a rock, knocking him out unconscious. With Diego out of the way, Soto turned his attention over to the mammoth.
Soto was about to leap in for the kill on Manny, until Diego stood in the way and refused to let Soto harm the mammoth. Without any second thought or regrets, Soto struck down Diego, severely wounding him. Before he could kill Diego, Soto then heard Roshan's cries and saw Sid with Roshan and walked towards them to have his revenge. Manny struck the distracted Soto aside with his tusks, flinging him into a stone wall, the impact causing several sharp icicles from the top of the wall to fall on Soto, stabbing and instantly killing him.