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In Exalted, everyone has a trait called "Compassion". People with high compassion are compassionate, caring. Solar exalted, those with the most extremes related to virtues, may snap and undergo the Limit Break "Red Rage of Compassion." It's angrier than the Limit Break "Berserk Anger."
Can also happen with Infernals who take Kimbery Charms. They can be hugely naive and unable to believe anything bad about their loved ones, but as soon as they personally witness their treachery or succumb to Kimbery's Torment, they will add that person's name to a potentially infinite list of "people who need to die slowly"...then do it. Kimbery herself doesn't really qualify, though, since her unique madness makes her a Mood-Swinger mixed with My Beloved Smother rather than this trope.
In Magic: The Gathering, the color blue works like this. It usually starts slow, but if it makes it to mid-game, your opponent is in VERY dire straits as he begins seeing his spells coming out wrong, his cards being discarded, his creatures taken control of, and his entire game plan obliterated.
There's also White, the colour of Law and Order, of healing and protection. And of 'Wrath of God' and 'Day of Judgement', which are the magical equivalents to a scorched-earth policy against anyone stupid enough to be a credible threat....
Of course, if you want a literal scorched-earth policy in white, there's always Armageddon - "Destroy all lands." See, white is all about equality—that is, destroying all of its own things and all of yours, no matter how many "all" was for you and how many "all" was for it.
Gennifer Weathermay-Foxgrove, from the Arthaus Ravenloft product line, is the more ladylike, gracious, refined, and patient of the W-F twins. She also has a kickass Berserk Button moment in Van Richten's Guide to the Mists, not to mention she's probably an unwitting werewolf.
The Children of Gaia are the gentlest tribe in Werewolf: The Apocalypse - but holding in that Rage all the time makes it that much worse when they finally lose it.
Demon The Fallen has traces of this as well. The Fallen were angels once, after all, with all the inherent goodness that usually implies. Even before the Abyss turned them evil, the Fall and the War of Wrath made even the gentlest among them into engines of destruction.
Promethean: The Created gives us any Promethean, potentially. They can follow several different Refinements, paths that guide them in their quest to Become a Real Boy. Usually, it takes months to go from one of these paths to another... except for Stannum, the Refinement of Tin. Stannum focuses on taking revenge on the world around you and on those who have wronged you, and it can taken instantly (though it takes twice as long to leave it). And most commonly, a Promethean enters it when they tell themselves, "Enough."And then the lightning bolts start flying.
In the Warhammer 40,000 fluff, the Salamanders, Ultramarines, Space Wolves, Blood Angels, and Raven Guard chapters of Space Marines are renowned for actually giving two-flips about civilians and their safety, and will ensure to avoid inflicting "collateral damage" unless they genuinely have no other choice. They are, in turn, hulking armies of nine foot tallFlame enthusiasts, Space Romans, Space Vikings and Space Psychos, respectively, who can and will ruin your day with all sorts of mayhem if you piss them off.
The Salamanders have loved ones (they still keep ties with their families on Nocturne, darn near unique among the canon Space Marine chapters), and they are all quite handily proficient as artisans and craftsmen, which qualifies for "art, culture, and hobbies." Whether they satisfy the textbook definition of "nice" is really a YMMV to be sure, though.
Blood Angels actually paint, and seek artistry as a form to express themselves, such was Sanguinius's view of life that a Blood Angel should always seek self improvement, which is one of the core beliefs of the chapter, to bring a better life to mankind.
Some members of the Ultramarines Second Company actually befriended the rest of the human forces of Damnos. Though they do not keep contact with them, those families that have one member inside the Ultramarines chapter actually are elevated in status.
Space Wolves almostwent to war with the Administratum for their actions after the First War of Armaggedon, where they decided to kill the entire human population of the planet because they fought AGAINST the demons. Not only that, but Logan Grimnar from time to time launches relief crusades that seek to protect Imperial citizens from the Imperium itself.
The Tau collectively as a civilisation have defeated many enemies that should by all rights have annihilated them. First, their homeworld is stuck in a sector of space full of Ork-held worlds, so they've had to deal with numerous Ork WAAAGH!s throughout their history. Then the Imperium launched a huge crusade to exterminate them, only for the Tau to fight them to a standstill at Dal'yth and force a crease-fire. Then Hive Fleet Gorgon hit the Tau Empire and they subsequently destroyed it as well. And then they defeated an even bigger Ork invasion fleet that threatened the survival of their entire empire. They've even held back daemonic incursions and killed a Daemon Prince, despite suffering from a complete lack of the special anti-daemon weapons needed to fight them effectively. This is coming from a small, relatively peaceful, and racially tolerant empire of only a few hundred worlds (compared to the Imperium, which has millions of worlds), populated by aliens who have even worse physical strength and senses than humans. For the Greater Good, indeed.
The original Traveller rules had a humanoid alien race, the Darrians, who were an easygoing, peaceful, slightly hedonistic culture. The neighboring, more violent, races generally left the Darrians alone. This was because the Darrians had stumbled upon an important discovery: how to make an ordinary sun go pseudo-nova. No one messes with the one who holds the Star Trigger.