"You are right," he began. "You did come to me looking to score, we did get fucked up and we did have a good time," assuring Nikolai that part of the story was true.
"And I did not savagely feed from you, it was to satisfy you. We were so high I wanted to have a taste, knowing you wouldn't notice anything but pleasure," Nikolai was old enough and aware a vampires bite could be pleasurable or painful.
"But I overfed," not commenting on Nikolai's stupidity remark, "and you started to overdose and quiver from lack of blood. I gave you some of my blood to bring you back, I think I was smiling, encouraging you... but then you started to puke, hard, and you started to choke. I pushed on your stomach to force more up to clear your throat. When that didn't work I tried to stick my fingers down your throat to clear your airway, still choking on your vomit and my blood and then you just relaxed like you were gone," it was a simple fact. Nothing more.
"I left the room and when I felt the connection I panicked and left the area, left you alone," some of that was true. He left the room but he did not feel bad leaving Nikolai alone and presumably dead, but he did panic when he felt the bond, that was true.
This was one of the rare times Mitch told the truth, present in the back of his mind that Nikolai could make Mitch show him the memory, to verify his story. Yet Mitch was not sure if Nikolai knew they could share memories of the past with one another, chess not checkers was the game. He needed his trust and if he had to tell the truth this would be the right moment to do it. To gain his trust so the experiment could begin.