Is it permissible to curse the soul of a deceased person if they were known as a Muslim but their actions contradicted that?

It is not permissible to curse a specific Muslim unless one knows with certainty that he died in a state of disbelief. The preferred view among the Muslim community is not to curse anyone specifically, whatsoever. As for cursing a specific disbeliever, there is no harm in it if he died in a state of disbelief. However, it is more befitting for a Muslim to avoid cursing altogether, due to the Prophet's (peace be upon him) saying: "The believer is not a slanderer, nor a curser, nor an obscene speaker, nor a foul-mouthed person."