Comic book superheroes have a near-endless supply of supervillains, scoundrels, and all-around bad guys to punch in the face for the sake of good and the protection of the innocent. Sometimes these threats come from distant worlds or alternate realities, but other times evil comes from the actions taken by our most beloved heroes.
Often times, a hero's rival will actually have their villain backstory closely tied to the hero themselves, with their (usually) well-intentioned actions having "made a monster". This makes those villains all the more compelling, as their fates are very much bound with that of their nemesis, and their rivalry is all the more exciting for it.
Of course the superheroes never really set out to create their worst enemies, despite what many news publishers in their universes would say. Most of the time, heroes don't even realise until too late the monsters that they have unwittingly sent out into their worlds, and begin to unleash absolute havoc.
Both Marvel and DC have dozens of villains that have been created by superheroes. These are just a few of the most diabolical supervillains created by their rival superheroes.
10. Namor And Black Panther (King Tchalla)
One of the earliest Marvel Superheroes and a popular Fantastic Four enemy, the king of the hidden city of Atlantis has always been a rash and violent man, often doing what other heroes and villains could not or would not do. And this often would bring him into conflict with other nations and their rulers, such as the great king of Wakanda, King T'challa, aka the Black Panther.
Namor and T'challa have been seen in the comics as allies, friends, and also rivals for decades now. Their own version of a cold war has been brewing between their powerful and technologically superior nations, until their rivalry reached its boiling point in the Avengers vs X-Men comic event.
In this worldwide conflict between Earth's mightiest heroes and the oppressed Homo Superior race, Namor was infused with a small fraction of the Phoenix Force's power, which allowed him to flood Wakanda with a mighty tsunami, killing countless innocents. This of course sparked an era of hatred between himself and the people of Wakanda, including King T'challa.
And while there were bigger threats to deal with at the time, King T'challa swore that someday he would find his vengeance against the man who drowned millions of innocent men, women, and children.
So during the buildup to the second Secret Wars event T'challa, with the help of the Inhuman King Black Bolt, stabbed Namor in the heart and trapped him on a dead planet where a massive bomb went off. The bomb killed Namor alongside Thanos and several other Marvel villains, finally bringing an end to the blood feud between the stubborn kings.