Sabretooth goes against Mentalo, who had the upper hand with his mental powers. Sabretooth decides to unleash his primal side, given that Mentello told him his mind is his weak point, so Victor Creed decides to lose his mind instead. He was successful in defeating Mentallo and saving the rest of Weapon X.
In the end, he reverted back to his animal trait and presumably, his good guy persona that he earned after the events of AXIS is now lost. If Sabretooth comes back in future stories as a bad guy is anyone’s guess.
From – Weapon X Vol. 3 #27