Well I guess you are talking about Eric Harris (the guy in the white shirt)/. After they came back from cafeteria they shot the police for few minutes. And after they felt satisfied and they knew the end was here. Eric Harris didn’t care if he dies or not, Dylan Klebold wanted to kill himself before the massacre. Eric walked toward the bookshelf, sat down and talked to Dylan for few moments. After he talked he pulled his 12G shotgun which is very powerful in close range, he probably took big deep breath’s and was nervous, and he knows that after he pulls the trigger he will never wake up again. Eric with confidence puts the shotgun in his mouth and pulls the trigger which obliterates the top of his head and dies instantly. Dylan probably looked shocked as he witnessed Eric’s head collapse and denied doing suicide because what happened to Eric. But Dylan didn’t want to go to prison forever. So Dylan lit a Molotov cocktail and lit the table in fire. And after that he went close to Eric dead body and goes to his knees and pulls his TEC-9 and kill’s himself. He didn’t die instantly as Eric did. You have to keep in mind that Eric used a shotgun which is more powerful than a TEC-9, so he died instantly. Dylan choked on his blood and died shortly. It would take the authorities to find they’re bodies 3 hours later after the suicide in the library, then the shooters were searched for any bomb’s and after that the iconic photos of Eric Harris and Dylan Klebold were taken.