At that point, I'm stunned. . . . I don't say another word. I simply look at him for a second and turn back around to my buddy and exit that conversation. . . . what would you have done?
You did the proper thing at that point - exited the conversation. I would have probably exited even earlier.
Reading your account of what he did - running a line across an opening that was being actively used by other divers - I conclude that the Instructor was a clueless idiot who only sees the world as it revolves around him. In fairness, we have only one description, and one side, of the story. It was inconsiderate of him to run a line where he apparently did. But, inconsiderate behavior is so common that it is hard to say that this particular instance was egregious.
I 'feel your pain' with regard to lines. Our local training quarry has experienced a proliferation of lines - of all thicknesses, colors, materials, and degree of tension - over the years, When I started diving there 12 years ago, there were a few lines connecting specific underwater objects. Now, swimming in the quarry reminds me of the scene from the Sean Connery movie, Entrapment, where Catherine Zeta-Jones has to navigate through a maze of laser beams to steal a diamond. I think many of the placements are both unnecessary and inconsiderate - they get in the way. But, I am also not about to cut them, with one exception - I scootered into, and became tangled in, a nearly invisible clear monofilament line that somebody had run across a large open area,
Last summer, I was teaching an AOW class in this quarry, which is not know for great visibility but was still remarkably clear for the time of year, and on the afternoon of second day I started hearing some serious 'thump, thump, thump' sounds while we were u/w working on buoyancy. Then, as we were swimming over to use a particular line - this is right in the middle of one of the most active OW training areas of the quarry - I swim into a cloud of sand and silt, while noticing that the thumping is getting louder. Turns out, some divers were practicing line-following drills in silt-out conditions, and creating the silt-out by pounding the bottom with a baseball base. To me, that was grossly inconsiderate because it necessarily interfered with the activities, and diving enjoyment, of a lot of other divers who had also paid to use the quarry. It was also completely unnecessary. These guys could have gone to one of the infrequently used, more remote, areas of the quarry, run some lines, and conducted their drills without bothering others. But, since the world apparently revolved around them, and they would have had to swim a bit to get to the more remote areas, they didn't. As much as I instinctively wanted to play Mike Nelson, and cut some second stage hoses, I didn't. I didn't even say anything to them afterward. It is part of living in a free society.
