A villain is forced to side with the good guys because of an inherent weakness
For most villains, a life of villainy is important to them. They seek out power, or attempt to further their own interests, and this aligns quite neatly with evil activities. Some become villains by crossing a Moral Event Horizon, while others are villains from the start. Sometimes, though, a villain will choose to turn good. Known as a Heel-Face Turn, this change can occur for many reasons. But what if the change isn't by choice? What if a villain has no choice but to turn good, to side with The Hero? This is the essence of an Achilles Heel Face Turn. It could be that the villain has an inherent personal weakness. If this weakness is sexual attraction, this may be a High Heel-Face Turn. When the weakness is sex itself, it is a Sex Face Turn. Alternatively, the villain may have a literal weakness, causing them to side with the good guys when confronted with a more powerful foe, not in the form of an Enemy Mine, but because the more powerful foe rejected our villain as being too weak. Occasionally, the weakness is imposed on the villain by another villain. It will often result in a Face-Heel Turn when the danger or weakness is removed. Compare with Enemy Mine, when a Heel-Face Turn is purely temporary to defeat a common enemy, with understanding that a Face-Heel Turn will follow once the common enemy is defeated. Be warned that most examples of this will be heavy with spoilers.