We traveled back to 1912 this week and met Sage, a woman who brought out all of the passion inside Damon. He used that to then light the same passion inside Stefan; i.e. feasting on humans.
This turned The Ripper loose.
In present day, Damon did all he could to try and wean Stefan's appetite, to control his brother whenever he drank from humans. It brought the brothers closer, but it also made for an awkward experience when Elena walked in on Stefan drinking from a woman.
Elsewhere, it turned out that Meredith framed Alaric only to then provide proof that he was NOT the killer. Why? BECAUSE HE IS THE KILLER! His ring has been making him go crazy, something that also happened to Samantha Gilbert, a relative of Elena's back in the day.