Why did The Flash kill off Ronnie Raymond (Robbie Amell) in season 2? First mentioned in the pilot episode, Ronnie Raymond was the fiancé of Caitlin Snow (Danielle Panabaker) prior to the beginning of the series, and one half of the nuclear-powered superhero, Firestorm. Due to an accident, Ronnie and Martin Stein (Victor Garber) became merged into one being.
Ronnie, who was thought to have died before the pilot episode, was revealed to be alive later on in The Flash season 1. It was discovered that he had been wandering around as a homeless man, while also trapped in the same body with Martin Stein. After being found by Team Flash, a way was figured out for them to finally separate, but also merge together again whenever Firestorm was needed. Firestorm left Central City but returned to fight side-by-side with Oliver Queen (Stephen Amell) and Barry Allen (Grant Gustin) against the Reverse-Flash (Tom Cavanagh) in the season 1 finale.
Flashbacks in The Flash season 2 premiere revealed that in the immediate aftermath of Reverse-Flash's defeat, the creation of a singularity threatened to swallow Central City. Ronnie and Martin had to become Firestorm and fly into the wormhole. The ensuing explosion killed Ronnie, but Martin survived. Robbie Amell returned briefly to The Flash as a manifestation of the Speed Force and as the Earth-2 Ronnie Raymond, but the season 2 premiere marked the end for the main version of the character. The reason for Robbie Amell's exit from the Arrowverse is linked to the CW's decision to create Legends of Tomorrow.
In the beginning, Legends of Tomorrow consisted of characters pulled from The Flash and Arrow. The plan was for one of them to be Firestorm. Robbie Amell has explained that his role on The Flash was "non contract", which meant that he could appear whenever he was available. Of course, joining the crew of the Waverider would have meant becoming a series regular. Though Robbie Amell says that the death of his character and his role on The X-Files (which happened shortly after) was "non-related", he still wasn't able to commit to Legends of Tomorrow, so a different character had to replace him [via Bleeding Cool].
Ronnie Raymond's death on The Flash amounted to an unfortunate (but heroic) end for one of the Arrowverse's earliest superheroes and an important Justice League member in the comic books, but one silver lining is that it gave rise to one of the Arrowverse's most memorable friendships, the one between Ronnie's successor, Jax (Franz Drameh) and Martin Stein, which developed over the first three seasons, and ended in tragedy when Martin died in "Crisis on Earth-X".