That is pretty vague. I think that the main question I have for you, is why do you feel a need to overcome it to begin with?
From what I understood, something bad happened to someone and you feel bad about not feeling bad about it? I don't think that the fact that something bad happen to someone means you need to change how you feel or think about them. I think you should be more sympathetic toward yourself and allow yourself the room to feel what it is that you are feeling.
That doesn't mean that you go to a funeral for instance and laugh at the widow. I think you can bring yourself to respect the codes of social behavior without changing the way you feel.
A friend of mind lost his father a couple of months ago, the man was very ill for years, and his death, in my opinion, a huge blessing, both to the father, but also, and mostly, for the son who could finally get back to his life without having to spend all his free time in the nursing home.
Even though I was really happy for the death of the father, I still went to the funeral and payed my respect. I told my friend that I'm sorry for his pain. A few days afterwards, not in the funeral, I told him that though his suffering now, I'm really happy for him and I think that this is the best thing that could have happened.
I don't know why you don't feel sympathy, maybe that person hurt you in the past, maybe you don't feel like they are being accountable to their own actions, maybe you just dislike them - I think it's important for you to stay with yourself and your feeling and not compremise them just cause you feel it's expected of you to feel something different.
I'd be happy to say more if you want to fill me in on the details.