SPOILER ALERT! If you haven't read the latest chapter of KHR (chapter 252, to be exact), you'll be spoiled! If you don't mind, continue on.
It's not all too often that Reborn feels regret and guilt like this. It comes with the job of being the best hit-man in the mafia. As a proud coldblooded assassin, he kills indiscriminately and remorsefully. Regret and guilt would only tie him down and weaken him.
But when Irie revealed the truth of why he was responsible for Byakuran, Reborn could not help but feel those two terrifying emotions gripping him.
It hit him like a cold wave washing over him, swallowing him and drowning him.
Irie said that the Bovino family had sent him shells for the ten-year bazooka and one had fallen on him, which resulted in him going to the future and crashing into Byakuran. But the only reason why the Bovino family had mailed him anything was because Reborn had shot at Lambo nonchalently and sent him flying into Irie's room. If he hadn't done that, then the Bovino family would never have sent them that assortment package of apology. If it wasn't for Reborn, this whole mess wouldn't have started.
If it wasn't for Reborn, Byakuran wouldn't have realized his own abilities.
If it wasn't for Reborn, they wouldn't have to go through all these trials and suffer tremendously.
If it wasn't for Reborn, then he wouldn't have to know that the Arcobaleno would be dead or dying because Byakuran wanted their pacifiers. (If he hadn't started this, maybe they would've all been happy and together. Maybe they would've defeated the curse and regained their original forms. Maybe. Maybe. Maybe.)
It was Reborn's fault. He felt the guilt gnawing bitterly at his heart. As Irie continued talking, Reborn's eyes slid towards Tsuna as if out of their own accord.
If it wasn't for Reborn, maybe Tsuna wouldn't have died.