I'm thinking of ghosting an ex who is potentially in a manipulative or abusive relationship and is contacting me
Why are you thinking of ghosting him?
I don't want to be involved in the situation
It is possible that your ex could look to you for help. I do not think that you should abandon your ex in their time of need potentially.
I don't care about them.
I understand that you are not obligated to be involved with your ex at all anymore, but this could be a life or death situation. How would you feel if you were able to help them from harm, but just simply chose not to? Perhaps you can help them just this once, then move on for good.