Trisha Cappelletti from The Most Popular Girls in School, is normally sweet, but if you hurt her friends, prepare for shit to go down. Poor Ashley Katchadourian and Shay van Buren learned this the hard way.
Yumeria in Card Players. There's obviously a reason she carries a gun everywhere she goes.
Flippy of Happy Tree Friends. Dear God.Background Flippy is arguably, the nicest character in the series, but he's also a former soldier who is so deeply broken by his experience that even the slightest hint of something that reminds him of war and/or violence causes him to turn into Fliqpy, an Omnicidal Maniac of a character whose only goal is to kill everyone else in the episode, usually in graphic and incredibly violent ways.
Steve the Pirhana Plant in The Movie of Bowser's Kingdom. Why After Hal insulted him one too many times, Steve told Bowser they had betrayed him, setting the events of the movie in action.
In Red vs. Blue, Sheila is pretty nice and even-tempered, especially in comparison to the rest of the cast. She's also the installed AI in a tank.
Another AI example is Delta. Mostly emotionless, he's nonetheless shown subtle and very touching dedication to his partner York, even insisting on staying beside him into death. He's later revived, and partnered with South Dakota, who proceeds to try using him as bait to let herself escape. When South is captured, it's Delta that recommends that she not be given further opportunity for betrayals.
Caboose is another example. He started out as an idiot and only gotdumber. However, he is by far the nicest member of the main cast; he cares more about making friends than fighting (unfortunately, when he does fight, generally he's a danger to his allies). However, when he worked himself up into a fury, aside from launching into a series of nonsensical one-liners ("Your toast has been burnt and no amount of scraping will remove the black parts!"), he defeated two armies of flag-obsessed zealots. And later quite a number of Texbots all on his own.
Doc was also a case while he was stuck with O'Malley for a while.
After spending most of Seasons 11 through 13 in an alternate dimension, he develops a split personality manifesting as O'Malley again. During their final stand against Charon soldiers he responds to Donut by saying "Aww that's so sweet" followed swiftly by "Now let us strike fear into the hearts of our enemies!" and an evil laugh.
Add North Dakota to the list as well. He's easily the nicest and friendliest of the Freelancers, to the point where he got the AI he did specifically because he has a nurturing personality and is the Team Mom. But he's still one of the top Freelancers, capable of mowing down an army of Mooks and expertly dual-wielding sniper rifles.
Agent Florida/Butch Flowers is this, despite his kind and joyful tone is still a Freelancer. Season 14 shows that He was more than willing to choose sim trainers as Cannon Fodder and took exceptional glee when Sarge and another murdered all of the Red Team Leader candidates.
Catbug is adorable, innocent and loves to play with anyone and everyone. He also shaved Danny's eyebrows (and possibly Chris' hair, too) upon acquiring an electric razor in "Browser Fail", and was revealed to have killed Jelly Kid upon their first meeting in "Jelly Kid Forever". (To be fair, he *is* part-cat, and happily justified his actions as being a present for Danny...like any cat would.)
RWBY takes place at a military school in a World of Badass, so any 'nice' character probably fits this trope. Special mention goes to Pyrrha, who introduces herself by saving a total stranger from plummeting to his death with her excellent spear-throwing skills and power over magnetism.
Jaune. In Volume 1, an Ursa attacked the Jerkass who was bullying him, so he ran at it and ended up decapitating it, and then topped it by turning around to the utterly gobsmacked bully and sternly telling him to leave his friends alone. In the season finale of Volume 2, an even bigger Ursa attacks him, and he slices it to ribbons.
In Death Battle, between two combatants, if one of them is nice and the other pretty much isn't, the nice one usually ends up winning.
And then there's the loud-mouthed, fun-loving Spider-Manslingshot-kicking Batman's torso off?
Goku vs. Superman is a battle between two incredibly friendly and kind, not to mention incredibly powerful beings. Goku is noted as being a family man outside his fights, and Superman is... well, Superman. Their clash results in the utter destruction of the Earth, and it's implied Superman was still holding back tremendously.