Niklaus Mikaelson made a lot of mistakes. It was just in his nature. His nature was to destroy. His nature was to never have redemption or happiness. His brother may have hope for him but in his own eyes, he was a failure.
There were times when his life seemed fantastic. Things were going well for him but them something would go wrong. Something always did. It was always the same thing! Constantly! He was betrayed by someone he loves. His heart became hardened and tinted black. He no longer cared for second chances or mercy
The reason Niklaus Mikaelson kept his siblings daggered and in coffins was for one reason only; fear. He was scared of being alone. He was scared of them abandoning him so he found a way to keep them with him always. It was cruel and unfair but quite frankly? He simply just couldn't care. Not anymore.
Many assumed that Niklaus turned off his humanity long ago but it was untrue. He felt emotion just as most others do but was good at hiding it. He could not show pain and he could not show weakness
It wasn't his fault. No. It truly wasn't. It was Mikael's. The man whom Niklaus had believed to be his father, the man who treated Niklaus unjustly, the man who had beaten Niklaus and in turn, hardened his heart.
Inside Niklaus Mikaelson was a broken soul. There were too many pieces to be put back together but perhaps a collage grander than the original could be put together. The first few pieces were together by Hope. Hope brought by a beautiful daughter and somewhere he had a (mostly) stable home. For once in his life, Niklaus Mikaelson was truly happy.