Karma, it will get you in the end. Since the events of "Give Them Hell Kid" last week, Niklaus and Elijah Mikaelson have created one hell of a monster, in the form of the abused son Marcel; or the new monster blessed with the serum that made Lucian Castle indestructible- Marcellus. The Originals are headed down one explosive path toward this seasons finale.
“ I asked, begged, for your help and you left Davina to rot. You put down Lucien because he was coming for you, for your family, but the thing is you made Lucien. You turned him, taught him, and then abandoned him. And then you blamed Lucien because it’s always somebody else’s fault. But maybe you were the villains in his story. After all, that is the Mikaelson way, right? Kill, maim, torture, all in the name of family. Always and forever. I used to admire that vow. It gave me back the thing that had been beaten out of me: faith in those you love. I was a fool. Always and forever is just an excuse to do whatever the hell you want. Maybe it’s time someone put an end to that. ”
Marcel said these exact words to Niklaus and Elijah before his heart was ripped out from him, and he fell dead into the river where he had spread his fathers ashes long ago. What the Mikaelson men failed to realize is that Marcellus took the serum before his death, and thus was reborn like Lucian Castle.