Are prayers answered against oppressors who committed murder and abused, and is there a sin in cutting ties with them?

It is permissible for the oppressed to supplicate against their oppressor, for Allah has granted them that permission, and the threat of the supplication of the oppressed is severe; it is an answered supplication without a doubt, just as the Prophet, peace and blessings be upon him, used to seek refuge from it. Despite the permissibility of supplicating against the oppressor, forgiving them is better.