We've already seen Mike's turning point when he killed Werner Ziegler.
Last week and this week's episodes were about him getting over it and becoming who he really is.
Characters tend to have multiple "break bad" moments on this show. There is just too much evidence pointing to Nacho becoming a problem with Gus, and him using Mike to fix that problem.
Mike is a fixer. In BB, he shows almost no emotional connection to people, and justifies his murders as "fixing problems," or no-half measures. The only way to fix a problem is to get rid of it, period. No second chances. Right now, we see a Mike that is remorseful about his decision to kill Werner. Sure he did it, but he was torn up about it and punished himself because of it. The Mike in BB is not remorseful about his decisions, that's the change that needs to take place.
Also, Nacho has show time and time again he is the type of person Mike deals with, he is exactly the same as Werner. A good person, caught in a shitty situation, that breaks the rules for his family. Gus does not trust Nacho. The only reason Nacho breathes is because he is Gus' window to the Salamanca side of things, and dealing with Lalo. Eventually that use will run out, and Nacho will try some desperate play to get out like he did with Hector.