how did donetello die in tmnt
Well, from what we have read in the comic, Leonardo, Raphael and Michelangelo were on a mission to stop Krang from transforming the earth using an alien device called the “Technodrome.” As for Donatello, he was left at the home base running command, and Shredder knew about this.
When his brothers returned from their mission, they were shocked to see the lifeless body of Donatello laying on the floor. Not too long after, Splinter is seen holding Donnie’s body in his arms while everyone else is trying to cope in their own way.
Donnie is a mutated turtle with brownish green skin. Unlike all his previous incarnations, Donnie has a gap in between his teeth. Like most incarnations, he wears a purple mask where the tails of the mask are longer than any of his brothers, except for Raph’s. Unlike previous incarnations, he is tall and thin with brown/red eyes. His quiet and shy nature gives him a more professional look, which comes across as the strong silent type. He is good with machines and has a talent for making new technology for his brothers. But there’s one thing he hasn’t gotten right. getting April to notice him.
Donatello and his brothers were once ordinary turtles; when they were infants, they were adopted from a pet store by Hamato Yoshi. When their soon to be father walked passed a strange man, a Kraangdroid, he felt something odd about him and followed. Unfortunately, Yoshi gave away his position when he accidentally stepped on a brown rat’s tail. The Kraang found him and attempted to kill him, but Yoshi fought back hard. In the end, the Kraang dropped the canister of Mutagen they were holding causing the turtles and Yoshi to mutate. Yoshi, having to have stepped on that rat last, thus mutated into a humanoid brown rat by the name of “Splinter”, while Donnie and his brothers, after coming in contact with Splinter when he was human, mutated into humanoid turtles. Splinter knew they would never be accepted in society, so he took them to the sewers, made a home down there, and made the four turtles his children. Knowing the world would be dangerous and there would be a time where they want to explore the outside world, Splinter trained the boys in the art of Ninjitsu. Donnie and his brothers were named after great Renaissance artists from a book Splinter fished out of the storm drain.
He is usually depicted wearing a purple eye mask. His primary signature weapon are his (2) effective bō staffs. In all media, he is the smartest of the four turtles. He is listed as 4′ in height on the original action figure release  . Donnie often speaks in technobabble with a natural aptitude for science and technology. He is the middle of the turtles, and third-in-command. Like all of the brothers, he is named after a Renaissance artist; in this case, he is named after Italian sculptor Donatello. 
The first film plays up the relationship between him and Casey Jones; in all other versions of the TMNT, Raphael, not Donatello, is closest to Casey, except for the 1987 cartoon, in which Jones does not figure prominently. This is apparently because the scene in which Donatello and Casey bond over the fixing of an old pickup truck was based on a similar scene involving Casey and Raphael in the Mirage comics. Corey Feldman famously provides Donatello’s voice in the first and third live-action films, Ernie Reyes Jr. does all of the martial arts fight scenes and stunt scenes as Donatello, while Adam Carl filled in for the second movie during Feldman’s stint in rehabilitation. In the third film, Donatello is the only Turtle who is not tempted to stay in the Feudal Japan of the past, saying that he can’t live without technology. The films also portray Donatello as being close with Michelangelo, bantering with him during fight sequences, going off to the side when Leo and Raph are arguing, and in general hanging out in their spare time.
It closes with Splinter uttering he words, “My poor brave son.”
Yep, they killed him. The world was left in shock and disbelief. The Saturday morning line-up that shaped our childhood existence was forever changed. It happened and it happened quick. The question was, who did it? Shredder? Krang? Slash? Bebop? Rocksteady?
Donatello’s character design was also updated, giving him a gap in his teeth and a taller, leaner appearance than his brothers.
Donatello or Donnie is one of the two tritagonists (along with Michelanglo) of the Teenage Mutant Ninja Turtles 2012 television series. He is the second-youngest adoptive son of Splinter, younger biological brother of Leonardo and Raphael as well as the older biological brother of Michelangelo. He is also the younger adoptive brother of Karai. He is the team’s scientist, mechanist, inventor and weaponry who is in charge of the design and manufacture of all the tools, gadgets, armor, vehicles and weapons in Turtles’ arsenal and is also a gifted hacker. Wearing a purple mask, he fights with a bō (rokushakubo) which also converts to a naginata (a Japanese halberd) via a switchblade inside one end of the wooden staff. In this version, Donnie has a huge crush on April O’Neil (who is also in teenage years in this adaptation), and later develops a rivalry with Casey Jones because of it.