Could you choose between the life of Cole or Solas if for some reason you had to? Lets say, that what Solas was doing wasn't evil but people didn't want the change, and Cole for some reason saw it fit to oppose him. Could you kill Cole yourself to save Solas?
Yes.
It would be a bit like shooting Legion or having Tali stab him in one of the torched ME3 playthroughs that I am never, ever visiting again.
For a moment, I'm fully in character and gun him down, obeying the 'cold calculus of war'. Shepard can feel sorry for Legion later, right now, Rannoch needs to be saved, so that the Flotilla will fight alongside the rest of the galaxy to save them all. One lovable synthetic's life is nothing compared to that weight on the scales.
The next, I'm myself again, hysterically sobbing and throwing the controller through the window and declaring war on Canada and getting the dogs really excited and barking, all of which vexes the cats.
If the change Solas was trying to bring about turned out badly, there would be words.