Ha Jinsung from Tower of God went and killed an entire branch of one of the 10 Great Families when they tried to buy the woman he loved as a slave.
Anak Zahard attempted this, when she entered the Tower to kill the entire Zahard family for the deaths of her parents at the hands of one of their members, her own aunt. She instead is soundly beaten by the first Zahard she meets and even ends up becoming her ally.
The catgirls in Something Positive. Apparently they weren't enjoying the convention; "They cancelled the fanfic panels. The con banned the sale of yaoi. The Twilight panel was nothing but heckling of all the books." The guy on the comics panel telling them their favourite comic sucked and Jason calling them out for overreaction when they killed him just pushed them over the edge.
Zeetha in Girl Genius. She was captured by pirates on her way to visit the wider world from her home, a "lost" city in the jungle, and her crewmates were killed. She slaughtered every pirate on the ship, shot down the rest of their fleet, and burnt down their stronghold. Only then did she realize she didn't know how to get back home and had just killed everyone who did.
Homestuck Vriska's "Make Her Pay" rallies everyone into rampage. Subverted with a punishing smackdown when she gets into the act.
Rose goes on one after discovering her mother's death.
Even more so after watching John getting run through by Bec Noir.
PM, after watching Jack kill pretty much everybody she ever cared about.
Kanaya goes into one following her death and return. It takes her about twenty seconds to disable two of the trolls she's fighting and introduce the third to her chainsaw.
Asperchu has a very sad one. The creature takes Sonichu and Asperchu to the Twin CWC Towers and tells Sonichu his father is here. He automatically assumes Ian Brandon Anderson and starts to hunt him down. The trope is played with when a) his Rampage of Revenge tears through people he once called allies; b) his attack destroyed the towers, something Ian is blamed for; and c) Ian wasn't the one Argentum was talking about - it was Asperchu!
In Rusty and Co., the princess goes on one when her wight boyfriend is turned.
Fighter in 8-Bit Theater goes on one against Lich after Black Mage is killed by Lich.
In Niels, the titular character, jealous of Agent 250 for the attention he receives from Agent 300, shoots Agent 250. In the stomach. At point-blank range. In front of Agent 300. The death glare that Niels fails to notice from 300 is the only clue we get to the rampage, as the next panel deems it "too violent to show". We do however, see the aftermath of the rampage: Niels has had his eye shot out with his own gun, Duncan's nose is broken, and a number of Niels' mooks are filling the other hospital rooms with undisclosed injuries, necessitating that the still-alive 250 share his room with Niels. 300, to his credit, comes out relatively unscathed.