He might have a good reason. It might not be exactly what you think. Sometimes I think the bioware writers are more clever than we give them credit for, but there isn't a way for them to do a lot of explication in the game. Did they end up leaving something out here?
Wow! I loved your thinking on how Quinn would do this, it also makes perfect sense. It seemed way too odd that Quinn would betray you, even more after all that you do for him. I remember when that battle started I destroyed the droids and him very easily. So easily in fact I was more angry his betrayal wasn't more of a challenge. The way you put it makes perfect sense, it gives me a renewed respect for Quinn as I believe from leaving Belmorra to leaving Voss he is one of the most loyalist companions you have.