A higher-ranking commissar can kill a lower-ranking one on the spot, IIRC. Of course, what constitutes rank in the commissariat is...fuzzy. It seems to be a seniority thing.
Of course, a commissar witnessing heresy in action has not only the right but the responsibility to execute the offender, be he grunt or lord-general. The implications of killing a general are, however, significant.>>13317309
Idolatry would be a serious, serious crime by Imperial standards.