When does the hero become the villain?

Right, this is likely going to be a longer series of posts, and possibly something more, as I find the topic genuinely interesting. It's a bit of exploring the topic of evil, virtue, heroism, things of that nature. 

Without going too far into context, I got an interesting question of when a hero would turn into a villain, and, in this particular case, that's a really good question. Obviously, I'm not talking about obvious cases like stories where "power corrupts", or "hero finds a cursed artifact" or any of the obvious "babyface turns heel" moments. This is more of a case where the hero never truly changes their moral code, and yet perfectly reflect the whole "While fighting monsters, take care not to become one" maxim.

 So, what's a good way of showing that the hero has, in fact, become the villain?

One such way would be to do it through repeated cycles. For example, we could have something like the following:

In Part 1, the Hero is some sort of a young rebel, protesting against the system, perhaps well enough to deserve being permanently cycled. They're down on their luck (as heroes tend to be in Part 1), on their back foot, hunted by the Tyrant's agents; perhaps, for some reason, the Tyrant's most valuable agent decides to be the one to bring the Hero in (or silence them, whatever), and, completely by accident, the Hero defeats the bad guy, and, even more accidentally, kills them. Perhaps it's the first act of real violence the Hero ever commits. The Hero is shocked, shook, confused, overwhelmed with conflicting emotions; the focus stays on them and their thoughts, feelings and allies. In the passing, a state funeral may be mentioned, alongside a memorial parade; the Tyrant, naturally, swears vengeance, placing the group in even greater danger - however, the group now also knows they can fight back successfully, and so do their potential supporters.

Now, imagine if much later, in Part 3 of the story, for example, the Hero has replaced the Tyrant and tries to rule. Perhaps they are not as successful as one would hope, and there are some protests and rebels. In order to save the fragile order, the Hero sends their agents to apprehend the rebels. The Hero's right hand person, a friend, and someone the audience knows and hopefully likes, goes off to personally make sure nothing goes wrong. Something does go wrong, the Right Hand dies. Once more, the lens stays on the Hero, their sorrow, rage, righteous anger. The reader sees the grand funeral, the heartfelt speeches, the shaken friends. The Hero makes a promise in front of the entire nation of ending the threat of this rebellion and avenging their comrade - and heads off on the "final adventure" fraught with danger and intrigue. Being better suited for catching rats than the deposed Tyrant, the Hero finds the rebels in good time; however, their meeting does not end well for the rebels - the Hero makes sure none are left alive. It's framed as justified, risky, appropriate - just like the bad guys in John Wick, dying by the dozen for the death of protagonist's dog as the audience cheers.

Has the Hero changed? Not really. Are the acts indistinguishable from the Tyrant's earlier actions? Perhaps only because the Hero succeeds where the Tyrant failed. The text is intentionally selective, giving full context for the Hero and none for his opponents. This allows the framing do a lot of work - in one case, it's survival, in the other, it's terrorism; in one case, it's the Tyrant's repression, in the other, the Hero's justice. I won't lie, I'm inspired by real world press with this stuff.

So yeah, that's the idea. Naturally, it's best shown through repetition - similar events done by different parties, with similar results but different framing doing the job of the magician's assistant.