Truth in Television. Normally, adrenal glands and the brain's limiters make it so that you only use around 20% of your muscles and maintain a thought of self-preservation. Putting a loved one, child, or sufficiently close friend of somebody in danger is the fastest way to see the brain release said limits and let adrenaline kick in. All thoughts of self-preservation (and everything else that makes shy, meek people shy and meek) are overridden, and it is quite likely that several dozen of your bones will be broken (if you are lucky) before they let up. People who manage to enter Tranquil Fury instead of all-out anger are even more dangerous.
Bruce Lee was known for taking challenges from aspiring martial artists. During the making of Enter the Dragon, a Too Dumb to Live challenger snuck into their home and scared his two children, Brandon and Shannon. An enraged Lee put the idiot in the hospital with one kick.
Vance Flosenzier, who fought off a motherfucking shark when it was attacking his nephew. Not only did Flosenzier immobilize the shark and free his nephew, he then dragged it to shore so that park rangers could shoot it.
Sun Quan, leader of the southern kingdom Wu during China's three kingdoms period. When Cao Pi declared himself emperor of the powerful northern kingdom of Wei, Sun Quan agreed to act as his vassal. But when Cao Pi requested that he send his son Sun Deng to the north as a hostage (and therefore a guarantee of loyalty), Sun Quan cut off relations with Wei, declared himself emperor of Wu, and told Cao Pi to bring it on.
Quite sadly, Sun Quan eventually did outlive Sun Deng, who died of an illness. Based on some of his irrational and self-destructive decisions in the following years, it appears Sun Quan came noticeably unglued as a result.
Supposedly professional wrestler Kensuke Sasaki actually adopted Katsuhiko Nakajima, who debuted in the business in his teens, to act as this both in physical presence and through the Kensuke Office organization, which is essentially a "brand name" for their family (Kensuke Sasaki and his wife, female legend Akira Hokuto also have two biological sons) and affiliated wrestlers.
Paparazzi don't bother Johnny Depp's family at home. Why? Because he's told them in no uncertain terms that if he catches any of them sneaking around his home taking pictures of his kids, he'll bite their nose off.
He once swung at a paparazzo with a piece of wood for trying to take pictures of partner Vanessa Paradis while she was pregnant with their daughter.
Jody Plauchet was abducted and raped by his karate instructor. Jody's father Gary planned for 10 days to kill his son's rapist and did so as he was being walked through an airport terminal. Despite a clear cut case of 1st degree murder Gary Plauchet only plead guilty to manslaughter and served 5 years probation.
May 31, 2011, a bank robbery took place in Sarasota, Florida. As soon as the incident began, US Staff Sgt. Eddie Peoples placed himself between the robber and his young sons, then advised them to hide under some chairs and moved other chairs to cover them. Eventually the robber pointed his gun at one of the sons and said that if anyone tried anything, he'd shoot the kid. After the man left, Peoples followed him and used his own van to block the escape vehicle, then disabled the robber with hand-to-hand combat. In an interview with news sources, Peoples said "You don't threaten people's children."
This Ohio man who, after hearing his three year old son's screams upon falling into a well more than 40 feet deep, scaled the well's walls down to the bottom to ensure that nothing bad would happen to his little boy before the fire department arrived.
A man from Turkey saw his four-year-old son fall from a fifth story balcony and immediately jumped after him Pushing against the balcony to gain momentum, the man grabbed his son mid-fall, wrapped his arms around him and then turned his own body over so that he would hit the ground instead of his son. The child was left completely unharmed while the father suffered from a broken shoulder and ribs.
Stuart Chaifetz was worried because his autistic child behaved oddly in school. He put a recorder in the kids's clothes and found out he was horribly bullied and abused by the staff. He's now on a crusade to protect all kids in such a terrible situation.
A Chicago father held off the would-be kidnapper of his two-year-old daughter. Here.
Israeli politician and current Minister of Foreign Affairs Avigdor Lieberman once clobbered a kid who was bullying his son, and threatened to do it again if he did it again. He wound up apologising and paying compensation.