Rebekah screams when Niklaus shoves his hand into her fiancé's chest. Her face filled with horror, pain and anguish and he feels for his sister, really understands her pain, because even though Niklaus didn't kills his love, she is still long gone, left behind on that ship a lifetime ago. She clings to her twin, clings to Kol as she screams. Kol doesn't let her go, doesn't let her crawl her way over to the spot where Alexanders bloody body lay in a heap, his heart dropped carelessly to the dirt next to him.
Niklaus immediately tries to justify what he's done to Rebekah who is a distraught mess, yelling that he was protecting them from the hunters, the group which Alexander was a part of. Rebekah screams back in an anguished and angry voice that Alexander already knew what she was, that he was going to find the cure for her and he pales, seeing the rage in his half-brother. He is not the only one who is surprised when he doesn't dagger Rebekah right then and there. Instead, he growls out at her, deciding that leaving her to deal with her dead lover and to be in fear that he would indeed dagger her eventually for this would be a more deserving punishment.
The three brothers help their only sister bury him. She chooses to bury him beneath the church where they planned to be wed, in the place where she was to say her vows and become his wife. She silently cries, her tears trailing down her cheeks, as she places his sword with him in the grave, watches long after the hole has been filled in. She truly hates Niklaus, and he knows she shall likely never forgive their erratic brother for this. He has very much lost their sister for the foreseeable future and didn't even realize what he'd done would cost him dearly. Niklaus thought that no matter what he did, they would all follow, all come back to him. That their sister was overreacting, that she would see he was right, that she would just get over this. He acted as though he never hurt them, as though everything was about him and that they were the only ones who could do wrong. It would take years for Rebekah to even be in the same room as him for more than a moment, years until she would even speak to him.