The sweeter, gentler, more polite, more peaceful, and overall nicer a character is, the worse it will be for whomever is in the vicinity when they're subject to one round too many of Break the Cutie, or Dude, Where's My Respect?, a Rant-Inducing Slight, or hitting their Berserk Button or Rage Breaking Point. What was once a sweet and nice individual suddenly snaps and becomes something far worse than the Big Bad could have expected.
Think it's called Unstoppable Rage for nothing?
Although any Nice One can make you wish you were never born, there are certain tropes which act as a sort of force multiplier. Check any of the following tropes to see just what is most likely in store for whomever set off the Nice One: Badass Adorable, Death Glare (which should never be brushed off), Mama Bear (or Team Mom, or Papa Wolf, depending), The Quiet One, The So-Called Coward, Teach Him Anger (a suicidal idea with the Gentle Giant), and Tranquil Fury. Especially bad when dealing with a Technical Pacifist, and even worse when dealing with an Actual Pacifist (for whom this is not an unusual trait). Combine all at once and there may not be a minimum safe distance to escape their wrath.
More Truth in Television than you think - deferring gratification with regards to expressing one's anger tends to make the end result... less than pretty. Also, it's generally more frightening when a more mellow person starts acting up because it's so unexpected. Even when it's not scary, it can make a character feel great shame, it serves as a What the Hell, Hero? moment when they realize they were such a Jerk Ass, even the nicest person they know couldn't deal with their shit anymore.
Cultures that have a reputation for being polite, non-confrontational, and passive-aggressive usually have a history of being very warlike in the past; think Scandinavia, Canada, Minnesota, USA & neighboring Midwest States, or Japan. These cultures developed polite, non-confrontational, passive-aggressive characteristics to keep from killing one another off; push them too far, however, and they can and will remember their heritage.
See also Good Is Not Soft, when a normally affable character realizes that nice won't always get things done in a situation. They can coincide, if the realization and the outrage are triggered at the same time. An Actual Pacifist subjected to the same stresses will turn into a Badass Pacifist instead (unless this becomes a Wham Episode in which pacifism is renounced.) The results are quite similar if they Madden Into Misanthropy, the difference being the new misanthrope isn't so much a violent dynamo as a care free Jerkass.
This is not to be confused with a character being a Bitch in Sheep's Clothing (or even Face of an Angel, Mind of a Demon). While the latter trope's 'nice' image is usually just a facade for their genuinely callous personality, this trope involves a genuinely nice character submitting to a rare act of malice. Repeated subjection of a nice character to this trope, though, may result in Flanderizing them to such a degree that it's impossible to tell the difference.
Can also result from being repeatedly subjected to the losing end of the Misery Poker trope when their combined traumas far outweigh any single problem another character has because of feeling like a Butt-Monkey due to their very real and legitimate problems repeatedly being ignored.
The polar opposite of Is This What Anger Feels Like?.
Compare/contrast Knight Templar. Compare Beware the Quiet Ones if they are known for their silence more than their kindness. Compare Killer Rabbit for when you need to beware the cute ones. Compare Beware the Silly Ones when they're known for clowning around. Compare Silk Hiding Steel, for when you should beware the proper ones. May also act as an emotional power.
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