In the mafia there were certain rules rival families had long learnt not to mess with, the rules involved messing with some of the Vongola guardian's better half. This rule became known as the GOLDEN RULE.
Rival families had long learnt not to offend, or hurt Haru in anyway unless you wanted to be bitten to death by that cold hearted killer she calls a fiancé. This rule was learnt the hard way when the Motisi Mafia Families decided to capture Haru as a hostage in order to make a bargain with the Vongola. Unfortunately for them after Hibari found out that his fiancee was abducted he marched to their base and killed every one in his path, even before the rival family had the chance to strike a deal. After that incident, The Vongola rival families learnt not to involve Haru in any mafia rivalry.
The other person the Golden rule applies to was Gokudera. Sure he the Vongola storm guardian along with Bel, he was also the Vongola Decimo's right hand man, he was a genius. But none of those were the reason why the golden rule applied to him. One of the foolish rival families that leant this the hard way was the Cosa-Nostra family who thought using Gokudera as bait in order to get his boyfriend's attention was a great plan.
Unfortunately their timing sucked, because Mukuro was supposedly to take Gokudera on a date that day, and Gokudera had forgotten to tell the mist guardian the time and location of the date. The plan to use Gokudera as bait was an epic fail as Mukuro single handedly killed every single one of their members, only to ask Gokudera, "Haya-chan What time is the date going to be, do you still want to go on the date?." Of course they went on the date, only after an hour of Gokudera grumbling and insulting Mukuro, and Mukuro smiling at Gokudera, because apparently an annoyed Gokudera turns him on.
The last person the Golden rule applied to was Squalo, when a rival family decided to drug Squalo as well as cut his beloved hair, Xanxus ordered the Varia guardians to kill the rival family as well as their spouses, children and pet, Bel particularly had a great time. While Squalo complained that he didn't need saving as he was NOT a damsel in distress. Too bad he looks like a lady.
There were others the rule applied to but the three were on top of the list.
