Why the fuck would anyone think that Mike was about to back Nacho if it turned into a firefight? Mike was there specifically in case Nacho didn't hold up his part of the deal, so that if he went off script Mike would put a bullet in his head to make sure the Salamancas didn't take him alive.
No way he was gonna kill everyone there, or even just the Salamancas, if Nacho decided to open fire on them. I mean his fucking scope never leaves Nacho. What would be the end game for Mike here?
If he kills everyone tjere, then he has everyone left alive in Gus's organization, Don Eladio's cartel AND the Salamancas who are going to figure out real quick that's what happened and make sure he, his family, Nacho and his family and everyone related to them by even five degree dies horribly.
Suppose he kills the Salamancas, it's not like it's the end of their family, and now there's a full cartel war that's gonna be waged against Gus. Most likely, Gus would have his killed immediately and his head served on a platter to Don Eladio saying he never asked him to do that, etc..
Let's say that even if Fring decides to go to open war with the Salamancas, he still murders the fuck out of Mike for being so retarded that killed the Salamancas while he was there and forced him into a war he didn't want in the first place.
And why would Mike do that? He would have to go to his daughter, convince her right away that she and her daughter have to leave right this second and completely disappear, and for what? To save the life of fucking Nacho for no reason?
"Why was he sad at the end then?" Just because he might think it sucks that Nacho just blew his head off and he had to watch him do it, that doesn't mean he wasn't 100% there to kill him if he Nacho didn't hold up the end of his deal. If he had wanted him to survive, he could have helped while he was in Mexico or let him know. What did he do? Nothing. He was gonna let him die because 'It wasn't his call'.
I really don't know what show half of you are watching. The idea that Mike would open fire on the Salamancas or anyone there is beyond retarded and complete misreading of this scene.