Who says (said) there is (was) an error? Sounds pretty OK to me.
With more words and perhaps a bit more force, you could say,
Sean is both a coward and a psychopath.
Or,
Sean is at the same time a coward and a psychopath.
Or,
Sean is a cowardly psychopath.
Or,
Sean is, at the same time, a coward and a psychopath.
Or,
Sean is characterized by cowardice and psychopathology [in the sense of abnormal, maladaptive behavior or mental activity.
Or,
Sean is an odd mixture of cowardice and psychopathology.
Or,
I would characterize Sean as a cowardly psychopath.