John listened with growing incredulity, the anger bubbling up through the hurt, making his eyes hard- and then Sherlock added insult to injury by demanding he trust him.
As if he needed to be told. After being played for a fool. And then, he brought up the last horror, the one thing John was certain he wouldn't be able to bear. He snapped.

"No," he said, interrupting, unnaturally calm at first, though it didn't take long for the rage to slip past the tight, overly controlled set of his jaw. His hands balled into fists at his sides.
"No, you know what, Sherlock, no. You don't-- you don't get to just do this, alright? You don't. Do you have any idea--"

He cut himself off and shrugged Sherlock's hands off his shoulders irritably. Seething, feeling utterly betrayed and so, so stupid, he gathered himself together and glared right back into Sherlock's eyes.

"Do you know what I thought, when you kissed me, Sherlock? Did you even care what I thought? Did how I would feel, what I would think- did it factor in at all? Only been bloody beneficial? You listen, Sherlock, you listen to me. When you kissed me, I thought that you were play acting. And then, then, Sherlock, I felt guilty, for thinking that you could be so cruel and inhuman, for thinking so little of you, my friend, my best friend... no, don't even think about interrupting me! I felt guilty for even thinking you would do that, that you would play with our friendship, the only... the only real thing there is in this god-awful place-- no, I'm not done, I'm not, I'm not even half done, so you can save your aren't I clever safeguards and you can at least pretend you understand what you've done, first, because if you don't I am walking away, I don't care how stupid it is. And you can trust me on that."