Damon has been a vampire since 1800 something. He's old as hell basically. And he was a vampire who openly didn't have much compassion. He was a risktaker and sometimes would act before thinking. This is just who he was.

One night he was being weird per usual and exploring Mystic Falls. Not like he didn't already know the place by heart. But something felt different in the woods. Underground there was a cellar and he heard soft whimpers and weak gasps. A person

He speeded with his powers into the cellar, not sure whether to kill or save. But he saw you locked in a cage. You were a vampire. Weak, tired...and thirsty