Character assassination is a deliberate and sustained process that aims to destroy the credibility and reputation of a person, institution, social group, or nation. Agents of character assassinations employ a mix of open and covert methods to achieve their goals to cause their rejection by their community, family or members of their living or work environment.
- raising false accusations
- planting and fostering rumours
- manipulating information to present an untrue negative picture of the targeted person
- exaggeration or misleading half-truths about the subject's morals or actions
In terms of how character assassination can go wrong ... on it's own, it's not as psychologically hazardous in my experience as some of the other options on its own. It *can* be, if the rumours are particularly nasty and taboo (i.e. accusing someone else's character of being a paedophile) but normally, in and of itself, the rumour mongering isn't particularly problematic unless compounded with some of the other issues we touch upon.
This is probably because it's so difficult to pull off and because in the early stages the player can be quite involved with fighting the rumours and so the conflict can be engaging rather than disempowering. After all, there's a chance that the rumour which you start which should be canonically hazardous either seems absurd to the other players (whose brains power their character's opinions anyway) or just doesn't particularly bother them.
After all, players are motivated by their enjoyment and satisfaction (even if that means working through painful emotions), and if the target of the assassination is a character they enjoy having in the game then they will subconsciously find excuses to justify their own beliefs about the character, discarding rumours they don't like and embracing the ones they do.
Which means, unfortunately, that where a character assassination is successful it is often taken as a reflection of the *player's* own failings in character generation because they would be protected somewhat against the attacks if people liked having their character around.
This, again, needs to be dealt with on the meta level. The players need to understand and agree with the major social sins of the group in order to keep this as a viable in-game tactic rather than an accidental meta-game one. If someone is believed to be boon broken in the game, then the players may need to consciously kerb their own interest in the character enough to allow them to be properly affected by it.
The main problem that can be faced even through this tactic is that the final step in a successful character assassination is a virtually unplayable character. That's why they call it an "assassination" because, by the end of it, life isn't really worth living for that individual. No player likes to be targeted by a campaign which they cannot overturn and which ends in a lingering 'life worse than death' state where they need to retire their character ... especially if the other players don't believe they should be able to retire the character to 'avoid repercussions'.
Therefore the same techniques should be used to help the player through it and to remind the player that people are not targeting them, they're targeting the character. In this light, the player should be able to retire the afflicted character. After all, once you make an absolute win, there's no real need to keep the character around anymore, is there? It won't be fun for the player and anything else you do would be merely kicking the comatose puppy. (This isn't to say that a motivated player can't continue with the character in an aim to turn it around - only that it shouldn't be forced.)
If the player is expected to continue with their character so that they can't "avoid the repercussions of their actions" then ask yourself about the ethics of Your Own Actions. If it were truly about teaching the player, and you really thought it would work, then why wouldn't you be happy to let the newly taught player have a fresh start? And if it's not going to work, aren't there better ways to perform damage control or teach the player then make them the centre of a negative loop?
Confused, frustrated, and upset players flail wildly. That's not going to improve your game.
Character assassination shouldn't be used as a form of player punishment from other players anymore than a Storyteller should crush a character because they don't like a player's play style.
NB: If you want a player to see their own character's character assassination through, then convince them about how much fun it would be for them *as a player*. They get to be the moral of the story, get loads of spotlight, and have a conflict to fight against that will turn out very poorly for you if they win. If you don't want them to know it was you, say that you're impressed by the good roleplay that's coming from it and get any of the other players you know who sincerely like the plotline to also provide support to the player.
If you're happy to make it a rivalry, or your character is openly trying to assassinate their character, see if you can make a pact not to tell other *players* which is true and which is false. Yes, that pact benefits you the most but it also makes for a more interesting story and some players may be happy to do that simply to see who, if anyone, manages to figure out the truth of their targeted character's claims.