Most of the stories of Theseus's later years don't exactly show him being particularly heroic. He pretty much spends most of his time kidnapping unwilling women like Hippolyta and Helen. He even tries to help his buddy Pirithous kidnap Persephone, wife of Hades, Lord of the Dead. (How in the world did he think he was going to get away with that?)
So, the guy who was once thought of as the great lawgiver and unifier of Athens dies with a reputation as a total womanizer and kind of a stupid one at that. Theseus is definitely not the only Greek hero to go down this road. Heracles ends up dying because he's tries to ditch his wife Deianira, and Jason's life is destroyed when he abandons Medea.
It's pretty interesting that the Greeks included these types of stories in the legends of their heroes. Just like in the great tragedies that the myths inspired, Theseus and his fellow heroes are eventually torn down by their own actions. In a way, the ancient Greek heroes are a lot more realistic than many of our modern superheroes. (It's not like Superman ever even thinks about cheating on Lois Lane.) Theseus and his heroic buddies may be brave and strong, but they're also pretty darn flawed.