Since funerals are supposed to be a time for rememberance and you don't see the person in a favorable light (especially since it concerns your child), I suppose the best thing to do would be to say no. Even if it is for the sake of peace, there's no reason that you should have to fake your way through the funeral by saying things you don't really mean. Maybe you can offer a different way to help by proofreading a speech that someone else would read at the funeral.

You say he/she "will be missed"... just not by you!

It depends on how grave the offense was. If we're talking sexual abuse or violence, then you should pass on speaking. Every person deserves to have some kind thoughts shared about him/her at their funeral. If you can't earnestly deliver that it would be inappropriate to accept.

BUT...

If the offense was an insult or slight that isn't horribly grave, you should remember one important thing:

Funerals are for the living.

Your dead relative isn't alive to hear your words of solace, so you aren't doing him any favors. This is for your other relatives who obviously like and respect you. They will be turning to you for comfort and strength, and it would be nice if you could give that to them, even in spite of past wrongs. You don't need to lie during the eulogy--stick to honest compliments and decent memories. Talk about how he will be missed (you don't have to indicate WHO will miss him!). Talk about how much he meant to his wife/family. Talk about his contributions to the world. You can leave your personal history out of it.

Give it some thought and remember that death is a reminder that life is fleeting. Love and support your family the best you can, while you can.

First, I'd have to wonder why my family member asked me to speak when she knew how I felt about her husband, and figure it must just be due to grief and stress. Then I'd find at least a few redeeming qualities in the man and speak about those, although it would be pretty darn short and sweet!

I am sorry I would not be able to do something like that. When it comes to someone hurting my child in anyway then them ask for a favor HELL NO !! even though it is the spouse. You said that you did not like that person and that would be enugh right there for me to say NO!!
Besides I think that the spouse must know about the incident that took place and I can not see why they would ask you knowing how you feel, regaurdless of you being great with words. Suppose someone did except that job of speaking at someones funeral and gave an honest eulogy about how they really felt about that someone. OH MAN !! LOL all Hell could break loose.