The most important element of Ashiya Doman's charm is that he is incredibly pathetic. A man whose own inferiority complex came to life, went back in time and told him he would never overcome it. A human who becomes entirely inhuman, who blindly lashes out at the World, who doesn't possess a will or even an identity-- right up until he obtains it, and he becomes less of a wild beast and more of a human, and he dies because of it. He's a monster because it's easier for him, and because he knows that monsters are strong; he's been living in one's shadow for most of his life. It's very important that Seimei truly, sincerely believed in Doman, couldn't even imagine him falling to evil, let alone something as petty as jealousy-- and now, at the end of it, past the regret and the horror at his actions, the man called Ashiya Doman is left with the absolute, unavoidable knowledge that one day he'll become the beast called Limbo, and that the one man who truly believes in him will become his greatest enemy. And Seimei knows that the one person he could ever consider something close to an equal will one day commit atrocities simply for the sake of proving he's better than him.
Of course, Seimei knows that, and he still stops him from killing himself.