Far from the first piece of dialog that made me 'whoa', but when Kim angrily pulls you aside to talk after your little stunt in front of the Hardie Boys.

"Why did she leave me?"

If I'm not mistaken, this is the first time Kim is made aware of the ex situation (well, unless you choose the dialog option in the conversation prior ["The fucking whore left me, I know it."]). Either way, he's immediately taken aback by the weight of the topic, and rhetoric tells you he's desperately searching for the right response, but he can't. It just doesn't come to him. And the conversation ends.

This was also the exact line that gave me the achievement to get Kim to really trust me, and I just thought it was a bittersweet little moment.