...so THAT'S what sleeper cell activation feels like. Because yes, YES, LET'S TALK ABOUT THIS, because Xena is such an interesting lightning-in-a-bottle-case study! While I would never discount the work done by the writers, Xena as a show is almost perfectly positioned both historically and structurally to consistently explore that theme.
The first puzzle piece is that Xena was a syndicated show at the tail end of syndication's total dominance of a distribution model. For those too young to remember a time when ongoing plots and prestige dramas weren't the norm, syndication is big part of why older television shows almost entirely kept plots contained to one or two episodes rather than having them span seasons. See, when a show is syndicated, it is licensed out to individual television stations/affiliates to be aired as reruns. The individual station chooses when to air them and in what order, and whether to just skip episodes they don't like in favor of the ones most likely to draw eyeballs, etc etc. The more a show is licensed, the more money you make on it, so there is an incentive to make each episode standalone to make them appealing to each station by enabling them to toss on whatever episodes they like without it being a problem for the casual viewer. Also, before streaming, easy access to dvds and episode recording, and the like, a show could not assume that even its fans would have necessarily have seen every episode. "Catching up" was not an easy thing, and reserved for the most dedicated, doing shit like physically mailing bootleg tapes! Therefore, shows needed to have a consistent formula that didn't lock out the person who couldn't watch last week for whatever reason. Characters remained within more of a status quo. Xena is a "monster of the week" style show, like X-Files. I mention X-Files intentionally, because it was one of the first to really break that no-ongoing-plots structure, and that shift affected its contemporaries, like Xena, who also started to follow suit.
That alone doesn't account for Xena being so primed to explore those themes, of course. Even staying within the same fictional universe, Hercules (which Xena is a spin-off of) and Young Hercules don't even come close to Xena's complexity on the subject. But that's because Xena's premise is perfectly positioned to interact with those practical constraints for this outcome in a way those shows aren't. The status quo that syndication demands remain mostly in intact is that 1) Xena was evil and really good at it, 2) she is trying to do good in the world now as penance but can never undo what she has done. Every episode is about Xena trying to save people while dealing with the consequences of her actions as a warlord. The fact that she was evil cannot be changed or diluted nor can the fact that she must continue trying to redeem herself, otherwise the show is over or is unrecognizable to the casual viewer. But this is also an action show, sometimes cartoonishly so, so she must also be fighting consistently! The core spectacle is violence and the core story is why violence is often evil. There is an inherent tension there that the writers either needed to interrogate earnestly or ignore, and they chose the honest, interesting route. They gave Xena a costar who is innocent and principled but loves Xena, and had her always asking why and trying to understand how Xena could be that person, while being put under similar pressures herself. They had Xena continue to use the tools she has, including violence, for good ends, and wrestled with the answers as to why that was ok, why the violence she did then and the violence she did now were different—and sometimes decided they weren't. They showed Xena struggling with falling back into those old habits because they are seductive and easy.
If someone asked "are there so many episodes of Xena where you find out someone tried to get her to change her ways many years ago and failed because that is a really great standalone premise, or because violence as a tool and power and vengeance as motivators are corruptive and hard to stop using once you start," the answer is yes. The show is cyclical because violence is. But also because it is syndicated.
It's fucking rad and interesting.