Tenkai Knights

When a bad guy turns good. This usually makes for a good plot, for three reasons:

It lets the writer reintroduce the villain as a "darker, edgier" hero.
It reinforces a desired notion of the inherent goodness within people.
It prevents the Worthy Opponent from falling victim to What a Senseless Waste of Human Life.
There are also various in-story motivations for the bad guy to make the turn:

An encounter with an All-Loving Hero or gaining a Morality Pet.
Discovering that Being Evil Sucks or possibly that Good Feels Good.
An Enemy Mine situation leading to Fire Forged Friendship or The Power of Love in the form of Deliver Us from Evil or Love Redeems changing their priorities. Conversely mistreatment by their boss or their fellow villains makes them rethink their loyalties.
Realizing that they are a Noble Demon.
A Heel Realization, if they had never considered their actions evil or wrong in the first place.
They become friends with a hero after fighting them.
A case of Even Evil Has Standards, Evil Versus Oblivion, and/or Pragmatic Villainy being taken to their extremes: A more sensible, honorable villain becomes a good guy to stop another, much worse villain from doing something so horrible that they just cannot allow it.
A Sealed Evil in a Can has spent so long imprisoned that they come to realize that Being Evil Sucks and become Evil Redeemed in a Can.
The Token Good Teammate in a group of villains has had enough and just decides to become a more straightforward and "official" good guy.
A villainous mole is sent to infiltrate the heroes, but then they become the mask, turn into a hero themselves, and abandon evil altogether.
Someone refers to Thirty Pieces of Silver, causing the villain to realize that they crossed a line and try to set things right.
The Dragon realizes their boss is wrong and has a Heel-Face Turn, resulting in them keeping the Big Bad from escaping.
By definition, a villain who has genuinely undergone a Heel-Face Turn cannot be a Complete Monster, since a Complete Monster cannot show any remorse for their crimes nor can they seek redemption.

By definition, a villain who has genuinely undergone a Heel-Face Turn cannot be a Complete Monster, since a Complete Monster cannot show any remorse for their crimes nor can they seek redemption.

Sadly, it sometimes leads to Redemption Equals Death, and when it doesn't, someone still needs to draw their "Get Out of Jail Free" Card. Otherwise they may find the good guys unwilling to believe them; their conversion met with a Heel–Face Door-Slam (or they may "merely" find themselves Reformed, but Rejected). On the other hand, the bad guy may reject their chance at turning over a new leaf altogether, in which case it's Redemption Rejection.