What is the ruling on one who slanders his mother with obscenities, or threatens her reputation or with death, or points a pistol at her and fires, merely out of suspicion?

Allah has forbidden disobedience to parents and counted it among the greatest sins. The Prophet, peace and blessings be upon him, prohibited reviling one's parents, even indirectly. So what if, in addition to reviling them, one accuses them of indecencies and defames their reputation?!

This son must repent to Allah Almighty for what he has done, by abandoning the act, regretting it, and resolving not to return to it. As for taking money from the mother, there is no objection to asking her for it if the son is unable to earn a living and is poor. In fact, she is obligated to give it to him due to his need and inability. However, taking it in the manner mentioned in the question is not approved by Islamic law due to the blatant disobedience and mistreatment of the mother involved.