I'm not a parent, but can you imagine how psychotic Dragon would get if Sabo got hurt in his place. Like to some degree Sabo is worried that if Dragon were to die, the revolution would die with him, and Sabo was raised from a very young age to think of himself as expendable because of his parents, and that kind of thing can settle into the marrow of your bones and not go away no matter how much time passes. He would more than willingly stand in front of a brutal attack to defend Dragon, though Dragon is the last person in the world who'd ever need to be shielded. It's instinct for Sabo to protect, plus this is Luffy’s dad AND a member of the D clan. Dragon is important, Sabo isn't, so he'd happily take a hit in Dragon's stead. Queue his blood splattering across Dragon's face and Dragon just has a psychotic break because first of all he probably most definitely has ptsd, suddenly all he can see is Sabo crying against him because he can't open the gate to Grey Terminal, ten years old with half his face burned bloody and unconscious, and he just fucking snaps cuz that's not just a revolutionary or the chief of staff or his second in command that's his fucking kid and he's watched him grow up for the past twelve years, he can remember when Sabo barely reached his knee in height and now he's bleeding out because he, the child, was trying to protect Dragon, the parent. Nonono, that's not how this works and now Dragon has to commit homicide. "Would you kill for you child" he sure would! He knows Sabo can take care of himself but if he sees that blond brat fall in front of him he is going to descend to the lowest level of hell and bring whoever pulled the trigger with him.