The part about "the sins we carry" makes much more sense translated as burdens. I had no idea what Touko was referencing in the beginning. I also didn't really connect the retrovirus part as a reference to Kirie. It definitely takes more courage to live on with a bad illness than to take the easy way out and kill yourself.
Well, this is what i got out of the Kara no Kyoukai:
The movie is about escape. In life, we all escape pain in our own ways, whether it be through video games or music or sports. We escape reality temporarily and for the majority of us, that is enough. In essence, suicide is the ultimate form of escape, it is "flight" rather then just "floating". Some people are content with "floating" through life, without a true sense of purpose or direction. They live for the moment, so to speak. However, there are those who get a taste of what it is like to fly. What it really means to have a greater purpose, or perhaps, to get a glimpse of life from a high view. Once you know that feeling, you can never be satisfied with floating again. When you see the world from an overlooking view, you realize this simple fact: the world is so big. There must be something greater then our small, daily existences. Thats why everyone who does eventually jumps. Their perspective on reality, their view can never be changed back to what it was. They want to follow that dragonfly, even if it will cost them their life.