I was looking at the time line and I was thinking about what Rebekah said regarding what Marcel did the thing that Klaus would never forgive him for. According to the time line two events happened in 1910 Klaus daggered Kol and Mikael found him in NO. Kol implied before his death he did nothing to get daggered what if he was right?
What if Marcel brought Mikael to NO and framed Kol for it?
I kept running my mind through all things Marcel could have done that Klaus would never forgive him for. The only thing that makes sense is framing Kol. Think about it bringing Mikael down on Klaus is bad but not completely unjustifiable given the kind of man Klaus is or was. But the the one thing that is bad the one thing Klaus would not forgive would be throwing his own family under the bus.