This one is very straightforward. Solas is pretty damn furious if you let the Chargers die. Patrick Weekes said in the Rated NA podcast that his banter with Bull after letting the Chargers die is one of the few moments in the game where you truly see Solas angry.
To compare, if you let the Chargers die:
Iron Bull: So, you going to let me have it, Solas? Or do I get to wait and wonder.
Solas: What do you mean?
Iron Bull: We’ve got the alliance with my people. Given how much you love the Qun, I figured…
Solas: I might scold you? Berate you for your decisions?
Iron Bull: Hey. The Chargers died as heroes for the good of the mission.
Solas: I never said otherwise. The truth is, Iron Bull, you are Qunari. I cannot be disappointed in your decisions. As a mindless, soulless drone, you could never make any.
While it may seem like he is simply dismissive of Bull and has been from the start, if you look through the arguments they’ve had up to that point, Solas’s anger and judgement is directed at the Qun - not Bull himself (other than the role he plays within it as a spy). He disapproves of how Bull defends the Qun, but he doesn’t see him as being a “soulless drone”. In one of their earliest banters, Solas says “surely, even you see, Iron Bull, that freedom is preferable to mindless obedience to the Qun,” which means that he gives Bull enough credit to recognize that the Qun is a flawed concept. Whatever respect he had for the man ends when he lets the Charger’s die.
If the Chargers live:
Solas: You are not Tal-Vashoth, Iron Bull, not really.
Iron Bull: Well that’s a fuckin’ relief.
Solas: You are no beast, snapping under the stress of the Qun’s harsh discipline. You are a man who made a choice… possibly the first of your life.
Iron Bull: I’ve always liked fighting. What if I turn savage, like the other Tal-Vashoth?
Solas: You have the Inquisition, you have the Inquisitor… and you have me.
Iron Bull: Thanks, Solas.
It’s a significant gesture for Solas to actively offer his support to Bull. Up to that point, their banters have been primarily antagonistic arguments over the concept of freedom of thought. Solas approves of choice over mindless obedience. It’s also worth noting that if the Chargers die, Solas never initiates the chess game with Bull in the banters.