Spider-Man and Venom

The temptation to turn a popular villain into a hero can be pretty strong.  To do this, you need to make the two enemies stop wanting to kill each other.  Might not be an easy task because just saying ‘they get along now’ doesn’t work.  You have to put effort into the change.  So, what can help with this?

1. If this is your main villain, you need to replace them with a new threat.  They can’t just walk away or be defeated then come back to be friendly without there being a new antagonist.  Otherwise, you have a big hole in your story structure.  You can’t just throw anyone or anything in there too.  The new threat needs to be as strong or stronger than its predecessor in terms of danger and impact on the overall story.
2. A mutual enemy isn’t always needed to begin this new relationship.  This will have to appear at some point since the main goal of the story will be to overcome whatever the new threat is.  Yet, the former enemies might bond in a different way even before the newest big bad appears.  Maybe one searches for the other to get answers or some kind of item for their own use.  The former villain might show up for forgiveness if that can be seen as something they would do.  Fun to just play with the concept. Of course, this is fairly difficult to accomplish, so the ‘uniting against a common foe’ is the easiest path.
3. If the former enemies are forced to work together, they probably shouldn’t be getting along perfectly. They ex-villain will especially be prone to not rushing to help their new ally. Self-serving habits don’t disappear quickly, so they will have to be eased into acting like an actual hero. Grandstanding and not working as a team would be common as well, but not to a point where they should be losing with no chance of recovery.  It could be something as simple as getting in the way or not listening to a plan because they have a better idea.
4. Insulting banter and sarcastic mockery at failures is always a way to go.
5. A source of friction can, and probably should, be that these two characters will have different methods.  At least at first.  The ex-villain may start off cruel, violent, and prone to use their former methods for good.  This would make them an anti-hero, which could pit their morality against the established hero.  For example, they could argue over killing or the use of torture.  Part of this storyline would be the villain growing out of their darker habits and/or the hero stepping out of their own comfort zone.
6. If the history between these characters is highly toxic, you might have a lot of trouble selling the new alliance.  That doesn’t mean it’s impossible, but you’re going to have to explain why they aren’t holding grudges.  In fact, they might be doing that for a very long time until a big event causes them to rethink things.  Maybe the ex-villain nearly dies saving someone to show they’ve changed. It could be a conversation followed by an act of redemption.  This really depends on what happened because there are definitely some acts that cannot be forgiven.
7. There is a challenging option to have this alliance be one-sided.  The former villain might be forced into the partnership, so they aren’t really friends.  They will continue working together until they no longer have a mutual enemy.  Another possibility is that the former villain is working towards redemption by aiding the hero, but they are repeatedly denied any credit.  They will continue taking action in the hopes of earning what they want, but never get it.  It means they’re still ‘allies’, but only in that they are superficially working together.