Chocking someone to death may have looked cruel and unnecessary since Jon could've done it with only thrusting a sword through his heart. But Jon had his reason for doing that.
Killing the man with a blade would cause the smell of blood to linger which might make their presence known to the enemy. Jon didn't want others to know of their position just yet. Though choking the man had caused some sound, that had been dealt by one of his companions who was good at hiding noise. but hiding the smell of blood wouldn't have been that easy.
Jon set down the body at a side and started searching through the place and quickly found the keys.
After that, he didn't waste even a second and started moving. They didn't have the luxury to be slow. Who knows when some troubles will creep out from somewhere?
The good thing is the knight-class cultivator didn't stand guard on each gate of the cells.