Well, it's safe to say that nothing in Mystic Falls ever goes according to plan. In The Vampire Diaries "You Made a Choice to Be Good," Bonnie and Enzo were on a mission: get to Bonnie's safe house in upstate New York where they would hide the Maxwell Charter Bell and get the cure from Elena for Enzo to take. But, alas, Cade showing up in Mystic Falls put a huge roadblock in the couple's way: Stefan Salvatore. Stefan didn't just put a dent in their plans, he killed them. Bonnie saved Stefan on The Vampire Diaries after he did something unforgivable — he killed Enzo.
Stefan killed Enzo on The Vampire Diaries after Cade sent him on a mission to either kill Elena or kill 100 bad people. The devil also offered Damon the same deal, except he had to choose between killing 100 people or killing Caroline. Damon refused, and convinced Bonnie to wait just a bit longer to give Enzo the cure, and instead get it from Elena's blood and give it to Cade so that they could kill him. She and Enzo could still have their happily ever after, it would just have to be delayed for a little while. But, as Bonnie was getting the cure from Elena, Stefan changed the ownership of the house, which got Enzo disinvited in. He ripped Enzo's heart out in front of Bonnie just after she got the cure from Elena, and went straight for her.
Stefan swore he would kill Bonnie to get to Elena, and he was just about to when Bonnie stabbed him with the cure. Stefan is a human again, or at least, he's going to be, but that doesn't change the fact that Enzo is still dead. Bonnie saved Stefan from his deal with the devil, but having the love of your life killed before your eyes tends to make you a little hungry for revenge. Let's hope Stefan can survive her wrath.