Dick Grayson must always keep his anger at bay and that is no easy task to accomplish. It is true that Nightwing is one of the kindest and most compassionate heroes in the hero community, but it is equally true that he is also the most explosive and dangerous tempered hero around.
If Nightwing is seriously pissed off at you, then start preparing your will because, no matter if you are one of the strongest beings in the universe, there is no corner of the planet where you can hide from Nightwing's fury and you can be sure that when he finds you (when and not if) you will surely be killed if you killed one of his friends or someone in his family.
The Joker has witnessed this. He killed the second Robin and he was killed by Nightwing. This is a lesson he can never forget.
That said, it is obvious that Dick must always restrain himself when fighting a villain, especially if said villain has injured a friend or family member of his. Because, contrary to popular belief, Dick was not trained only by Batman.
He wasn't.
First, Dick was trained by Haly's circus and his parents from birth to become the best acrobat in the world (and he eventually became one).
Dick has learned to walk the tightrope since he was only three years old.
Both literally and metaphorically speaking.
Batman has been training him for almost a decade since Dick was eight years old. As he was the first Robin and obviously unable to predict that there would be others Robin, Batman had trained him as efficiently as possible.
Unlike the other Robins, Dick's training was very different and included many trainings that were not taught to the others.
Batman had trained him to take out all the members of the Justice League if he had to.
And Batman had also trained Dick to take him down.
As the first Robin, Dick had harder, more intense and efficient training than all the other Robins who followed him in the future.
The trainings of Jason, Tim, Stephanie, and Damian as Robin was nothing compared to Dick's.
But the Dark Knight was not his only mentor.
Not even close.
Lady Shiva was impressed with him when they first met and she volunteered to train him.
After a few years Dick accepted her proposal and trained under Lady Shiva, the deadliest woman in the world.
Deathstroke The Terminator was always his greatest rival and the only villain capable of getting under his skin. Deathstroke saw potential in him and initially practically forced him to become his apprentice before Dick was rescued by his friends. However, Slade always left his door open in case Dick wanted to train with him.
Dick, despite knowing what a bastard Slade is, accepted his offer since he knows that Slade would teach him things that no one else would be able (or want) to teach him.
So Dick trained under Deathstroke The Terminator, the world's deadliest mercenary.
The Court of Owls wanted Dick to be their The Talon, their Gray Son, the one who would lead the Court of Owls to conquer Gotham.
They not only tortured him but also brutally trained him, adding more lethal moves to his repertoire already full of them.
Above all William Cobb, his grand-father.
Dick trained with the fucking Justice League when he was Robin, trained with the Titans and with a multitude of aliens, Kryptonians, Amazons, gods, demons, Atlanteans, cyborgs and more.
No one in the world knows exactly how dangerous Dick can be.
Dick knows many certainties in life, and death is not one of them (see Ra's, Jason and the League of Assassins).
But the fact of which Dick Grayson is 100% more certain than his own life and of which no one else in the world is aware is the following.
He was molded to be the perfect weapon capable of killing anyone he meets.