My thoughts: let me show you them.
So, when talking about the Heart, something people bring up a lot--and rightfully so--is what it means about Corvo that he uses the Heart of his late, very dear friend and lover (or just the first if you don't subscribe to the notion of the second, though I do and will be writing this with that in mind) for the purpose of finding magic bones when being trapped there is torment for her. While you could say it's cruelty on his part (and, if you were looking strictly at high-chaos Corvo, you might have something there), I think of it a different way.
Specifically, I actually think it's tragic: I imagine him slowly figuring out what the Heart really is overtime, trying to deny it, but by the time he gets to Dunwall Tower and Daud that's impossible to do anymore. Imagine figuring that out in nearly the same day he's betrayed by people he trusted and someone he cares about very much is stolen from him. How powerless does he have to feel? How heavy is the sense of failure when everything he's been doing with the conspiracy has been aimed at avenging Jessamine and keeping Emily safe--and at that moment Emily is in Havelock's hands and Jessamine's tormented, disembodied soul is in his?
It seems like a culmination of the lack of control Corvo has experienced throughout his life. While we don't know much about his life prior to the events of Dishonored, we do know he was a diplomatic gift to Gristol from Serkonos, he can't be with the woman he loves publicly, and if Emily actually is his daughter, he can't acknowledge it. Corvo's life seems to be determined by the ebb and flow of politics--and losing Emily again and figuring out what the Heart really is might just be the mollusk that broke the whale's back.
Of course, that's when matters really get interesting, and Corvo finally takes his life into his own hands. What he does with the Heart after that is hard to say. A high-chaos Corvo might keep it, but as Dishonored's all about choice, I choose to recognize the low-chaos ending. (See what I did thar?) In that case, it's my personal headcanon that after everything's said and done, Corvo burns it. Give the last parts of Jessamine a proper funeral, after all.