Super sick chapter. This is another of those chapters where I constantly wonder what actually moves in Gwangnams head. Due his speculated inability to feel emotions...
You have to consider as well the context in which he said those things, in the way that he did. Deukchun is on front him listening, and Volcano. He's relaying a message to Deukchun (who's still conscious and listening) to say, this is what happens when you weaken yourself with drugs, and no one will there to protect you when things go south fast. And I believe him on his morality that he won't use force unless peace in areas of his daily life, and friends', are jeopardized. Eventually, that encompasses all of Kana, so Killberos is a sort of natural conclusion (threats keep coming from different rotten places of Kana otherwise).
It wasn't as much about feelings as it was reasoning. He's concerned I'm sure, but won't break his principles. He'll fight when absolutely pushed to, not to just have revenge, and that's not really being a bad friend imo. Other than revenge or not, he didn't display empathy to Deuk to teach him a lesson.
Also, as you said appearances matter, that's a way of preserving peace. It's a preventive measure when others don't dare to act out of line cuz of your established past, so he wants to protect Deukchun's Mad Dog image, that even those outsiders heard of.