That was fantastic. Thanks for posting all this great Wondy stuff, I don't think I would have ever become a WW fan if not for you :)
Ariadne, if you're curious, is the daughter of King Minos, of minotaur fame; she gave Theseus the ball of string that let him survive the labyrinth, but when he left Crete he left her as well.
In the version I read as a kid, he didn't just leave her, he left her on a desert island. Knowing full well that she would either starve to death or get devoured by some wild beast. She was saved by (and married) Dionysus, but Theseus didn't know that. So, yeah, I wouldn't call him a decent human being.
Granted, all "heroes" were assholes at the time, what with values dissonance. But still...
All the same, the grief-maddened Phthia was clearly still a danger to him and I can understand him not wanting to release her, even without his son's Wormtongue routine.
I can understand his need to get Phthia out of the way, but since he knew she was innocent, and the adoptive daughter of the woman he claimed to love, the very least he could have done was locking her in a nice cell. The fact that he left her into the "darkest, foulest, most pestilent underground dungeon" speaks volumes about what kind of asshole he was, even without getting into the whole "tried to have her secretely murdered" thing.