This is a good question. Let me begin with a summary and examine the potential humor in the story of Bacchus and Pentheus. This story occurs in book three of Ovid's Metamorphoses.
In this context, we read about Penetheus' desire to prohibit his family and others from worshipping the god, Bacchus. As others begin to worship Bacchus, Pentheus stands unmoved. People try to persuade him, like Acoetes, who is a convert to the worship of Bacchus, but no one is successful. In fact, Pentheus goes to Mount Cithaeron to spy on the devotees of the god, but when he arrives, his own mother and aunt kill him in a bacchic frenzy. They believe him to be an animal and hunt him down. More specifically, his aunt rips off his arms, and his mother decapitates him and cries out in victory. This is pretty gruesome stuff.
In the immediate context, there is little that is funny, but the broader context might have one humorous element. Actaeon, a member of the Cadmus family, as is Pentheus, is transformed into a deer as a punishment, because he saw the godness Diana bathing. What makes this story even sadder is that Actaeon's own hunting dogs kill him. In like manner, Pentheus is killed by his mother and aunt, as they believe him to be an animal. From this perspective, Actaeon is turn into an animal and killed by dogs and Pentheus is thought to be an animal and killed by his aunt and mother. There is something ironic about this all.