Warning! Spoilers for Teenage Mutant Ninja Turtles: The Last Ronin comic series below.
Michelangelo is the deadliest Teenage Mutant Ninja Turtle ever in the Last Ronin. In the Turtles' dark future, Mikey is the last surviving member of his family and in order to get revenge, he's become a cold-blooded killer. His kill count and violent acts committed in the series thus far have pushed the character into new territory.
Teenage Mutant Ninja Turtles: The Last Ronin by Kevin Eastman and Peter Laird has been a violent affair, with multiple deaths and murders that make the original Teenage Mutant Ninja Turtles comic run (which was violent itself, Shredder was murdered in the first issue) look like a bright and happy adventure. The dark future Elseworlds-esque story has shown how a potential film reboot should be handled. Michelangelo has transformed into the Last Ronin after seeing his brothers and Master Splinter die. He has one quest: Kill Hiroto Oroku, the person responsible for all the grief and death in his life.
Michelangelo was the perfect choice for the Last Ronin, as grief has completely changed who he was. He's seen too many close to him murdered and has become suicidal and plagued with PTSD after his family's fall. However, he eventually realized his destiny is to kill Oroku, Shredder's grandson, in an act of vengeance. Michelangelo's turn into a relentless killer in the series was highlighted by his slaughtering a whole group of evil men in the mountains who wanted to kill him for being what they called a monster.
It's not the only mass-killing Michelangelo committed, as the same issue shows him standing above a large pile of Foot Clan soldiers, as he states he wants to restore his family's honor.
Weapon-wise, Michelangelo has grabbed the iconic tools of his fallen brothers. He still uses his trademark Nunchakus as his primary weapon, but he also has Leonardo's twin katanas, Donatello's bo-staff, and Raphael's twin sai. As the Last Ronin, he's incorporated all of his (and his brother's) training into becoming the ultimate warrior against the Foot Clan. His kill count has got to be in the hundreds, if not the thousands.
The Turtles have been violent before, but they've never been full-blown killers like Michelangelo is as the Last Ronin. He's become the deadliest Turtle of all-time, one who won't stop until Oroku Hiroto has breathed his last breath. While it's weird to think of the Teenage Mutant Ninja Turtles most fun-loving brother becoming a stone-cold killer, in Last Ronin's dark future, he doesn't have much else to live for. He wants revenge and will continue to kill - and possibly die - to achieve his goal.