15He once beat the Joker to death
Just like Batman, Nightwing never kills criminals, regardless of how terrible their crime. Joker has tormented the Bat-Family for years, but in the series Joker’s Last Laugh, he pushed Dick Grayson too far.
After believing rel="noopener noreferrer">Tim Drake, Robin at the time
rel="noopener noreferrer">Tim Drake, Robin at the time, to be dead, Dick is distraught. The Joker relentlessly taunts him about it, begging him to hit him harder.
Nightwing continues to beat the Joker until Tim reveals himself. Dick realizes he isn’t dead, but it’s too late. He killed the Joker.
The Joker’s heart is restarted and he’s sent back to prison, but Dick can’t get over the fact he killed a criminal.
He’s ashamed he succumbed to hatred and revenge rather than justice and walks out on his family. It’s one of the darkest moments for Nightwing so far.