Well, you can 'escape', but in a slightly different way. After killing all of the people with riot shields, you go from outside to inside. He will take you over to the green door and go inside.
When you go in the door, walk down the steps and then turn around and run back out. They will keep going, but you are free to roam outside of the building, and inside the area where you walk extremely slow.
The firing squad barrier is still there, but there is a tutorial on how to get past them and run around behind them.

Just to be clear, I'm not taking credit for this. It is just something that I have done many times and know works.

Here is the original tutorial: You must login or register to view this content.
It shows how to escape at around :50 and the rest of the video is how to get out of the map. I don't know why it showed him being killed, but I think it is because he was close to them when the kill was supposed to happen.
Also, first post, I really like this place, and I hope I haven't done anything wrong in this post.