Klaus later finds out and compels Stefan to kill her, but when he is able to resist the compulsion, Klaus makes him turn his humanity and emotions off. However, he is later able to get his humanity back, which is seen when he saves Klaus's life to protect Damon, but yet he pretends not to care about Elena anymore.

How did Stefan Salvatore become human?

When a humanity-free Stefan arrived at Bonnie's house planning to kill Elena, the Bennett witch injected Stefan with the Cure, making him the first vampire to successfully transition into a human without drinking the Cure directly.

How does Stefan become human in Season 8?

Right after Bonnie gets the cure from Elena, Stefan rips out Enzo's heart right in front of her, leading to his death once and for all. ... Right when he swears to kill Bonnie, she stabs him with the vampire cure meant for Enzo, which turns Stefan human again.