TLDR Headcanon that Corazon/Rosinante fought tooth and nail to keep his kindness.
Corazon fought his circumstances, first as a Celestial Dragon who was encouraged to be cruel, second as a child who was hunted and hurt for his people’s crimes, third as a freshly traumatized boy whose life was defined by the hatred and violence of the streets, fourth as an orphan taken in by a vice admiral, and fifth as Doflamingo’s right hand man.
I mean, I see a good amount of content pointing out the horrors Rosinante faced in his childhood as he was persecuted by the people who he held no ill will against, but so often I see people brush off the trauma he must have faced growing up in the marines.
Like, you snatch up some poor, scared, hurting child, and drop them in the center of a marine base? Imagine what he must have seen. Imagine the feeling of being on the edge of shipping out for battle, the feeling of returning from a buster call with fresh blood on your hands, imagine the feeling of seeing this as the background of your adolescent life.
Being a marine crushes soft hearts, and has little patience for anyone willing to question their violence. Corazon would have been in an environment where any perceived “weakness” was a fault to be exploited or destroyed. So it’s easy to imagine that it would have been a conscious decision to stay soft. To stay kind.
Hell, life as a marine, much less as a child raised by and around marines, would have necessitated leaving anything as “useless” as kindness behind for the majority of their time. And I know that marines can be kind, and that many of them are; it’s just that Rosinante would have been raised in the heart of an organization that runs on wars. There’s no way that their calloused, cruel almost, ideals wouldn’t have gotten internalized.
So with that context in place, Cora would have had to fight to keep his bleeding heart. Every circumstance demanded that he become cold, but instead, against everything that normally would have happened, he didn’t.
Like, this is a guy who is defined by his actions. We remember him as the man who died to save Law. We remember him as the man who acted as his brothers Corazon. But more than this, we remember him as the man who went out of his way to make a sad, angry, child smile; and who was fully committed to ensuring that Law lived, even at the expense of his own life.
Cora has undoubtedly done some horrible, violent, things in his lifetime, but throughout it all he remained, at his core, kind.