It's one mistake.
One stupid, inexcusable, fatal mistake.
Damian should have known better. He should have seen it coming. He should have known that he wasn't ready for this; wasn't ready to be Batman. Wasn't ready to have someone else's life in his hands. Especially not Dick's life.
The life that was bleeding out in his arms.
Damian held his older/littler brother, cradling him to his chest, as blood redder than the Robin suit spread slowly across the boy's chest. And Dick was a boy, now, was only a boy, and he was losing blood faster than Damian knew what to do with. Damian wasn't prepared to deal with major medical trauma in the field. He felt paralyzed, clutching his dying big brother to his chest and wishing this had never happened, that he and Dick hadn't chosen to come out tonight, playing dress-up.
Because Damian isn't worthy, not of wearing the costume, being Batman's heir, but that's not the important thing right now. He's not worthy to be Dick's brother – not if he can't even save him from a bullet ready to spill his life's blood in the street.
At this point, Damian can't even move; he's trying so hard not to pass out from the overwhelming fear at the realization that this is his eldest brother; this is the only person who ever believed in him, who ever stood up for him, who ever acknowledged that there was something more in Damian's damaged soul than a killer. This is Dick Grayson, who for all intents and purposes raised him into the man – the boy, he's just proved he's not a man yet – that he is today. And it's Damian's fault that he's dying. When he is alone and there is no one who trusts him, who believes in him anymore, Damian will have no one to blame but himself.
The hand that lands on his shoulder isn't a hostile, but it's too heavy to be Tim. Which means, Damian realizes with a jolt, that it's his father. And here Damian is, holding the blood-soaked body of his father's eldest son; his father's true heir. Because he just proved that he is not now and never will be ready to take up the Batman's mantle; only Dick Grayson receives that honor.
His father's hands take Dick's limp figure from his arms, and then Tim is there, too, guiding Damian away. Damian just goes with the flow. He's lost and frightened and realizing for the first time just how awful it will be to live life without Dick.
At first he tries to keep track of where his father is taking Dick, but Tim is insistent.
"You're wounded," are the first words Damian registers out of Tim's mouth, but Damian doesn't care if he's wounded; Dick's hurts are far greater than his own. He wants to follow his father and Dick.
"You'd just be kicking yourself with the damage you caused," Tim says, somehow forcing him down onto a bench and ripping off the cape and cowl, taking off the armored chestpiece. "You'll have plenty of time to feel guilty later."
And as Tim cleans and stitches the meaningless wounds on his chest, Damian finally breaks down. The stress has been catches up to him all night, and the dam finally shatters. He doesn't even care that Tim steps back, startled, as Damian starts sobbing.
"I can't handle this," he sobs, "I can't, I can't. Change me back, please."
Tim's eyes are bugging out of his skull as he watches this display, and Damian can't say he blames him.
"I just want to be a boy again."
A/N: Yes, the ending is supposed to be ambiguous. What do YOU think happens?