I was brought up watching pantomime too, gnomad and it's clear that this series is not going to be of interest to children unable to understand that it's pretend.   Folktales -- on which pantomime depends have clear messages. The ogre/giant/evil-stepmother gets killed/severely injured because he/she is bad. It might be an extreme form of punishment but it's shown as just desserts.

There is no reason a series like Doc Martin needs a message. It's not that kind of show. Black humour can be my favourite kind -- but I believe the ending of this episode was played for cheap and nasty laughs. And I didn't like it.
Cheers, Joy