Theseus was a descendant of an Athenian King He is known for killing the Minotaur after being helped by Ariadne-Isis. He promised to marry her and took her away but abandoned her on an island. She was found by Dionysus-Osiris.
Theseus was also known for being inspired by tales of his ancestors who had to go out and kill evil people or animals who were killing and torturing innocent people. He went on a quest to be a vigilante and did in fact kill some very evil men.
Some of his labors include the following per Wikipedia:
At the first site, which was Epidaurus, sacred to Apollo and the healer Asclepius, Theseus turned the tables on the chthonic bandit, Periphetes, the Club Bearer, who beat his opponents into the Earth, and took from him the stout staff that often identifies Theseus in vase-paintings.
At the Isthmian entrance to the Underworld was a robber named Sinis, often called “Pityokamptes” (Greek: Πιτυοκάμπτης, “he who bends Pinetrees”). He would capture travelers, tie them between two pine trees that were bent down to the ground, and then let the trees go, tearing his victims apart. Theseus killed him by his own method. He then became intimate with Sinis’s daughter, Perigune, fathering the child Melanippus.
In another deed north of the Isthmus, at a place called Crommyon, he killed an enormous pig, theCrommyonian Sow, bred by an old crone named Phaea. Some versions name the sow herself as Phaea. The Bibliotheca described the Crommyonian sow as an offspring of Typhon and Echidna.
Near Megara, an elderly robber named Sciron forced travellers along the narrow cliff-face pathway to wash his feet. While they knelt, he kicked them off the cliff behind them, where they were eaten by a sea monster (or, in some versions, a giant turtle). Theseus pushed him off the cliff.
Another of these enemies was Cercyon, king at the holy site of Eleusis, who challenged passers-by to a wrestling match and, when he had beaten them, killed them. Theseus beat Cercyon at wrestling and then killed him instead. In interpretations of the story that follow the formulas of Frazer’sThe Golden Bough, Cercyon was a “year-King“, who was required to do an annual battle for his life, for the good of his kingdom, and was succeeded by the victor. Theseus overturned this archaic religious rite by refusing to be sacrificed.
The last bandit was Procrustes the Stretcher, who had two beds, one of which he offered to passers-by in the plain of Eleusis. He then made them fit into it, either by stretching them or by cutting off their feet. Since he had two beds of different lengths, no one would fit. Theseus turned the tables on Procrustes, cutting off his legs and decapitating him with his own axe.