Walt's livelihood is threatened once again when one of Mike's guys decides to flip.
It's up to Walt to save the day. He needs the name of all of Mike's "employees," who have been confidential up 'til now. Even if it means he needs to shoot Mike (with Mike's own gun) to get them.
It's not until he has mortally wounded the man that Walt realizes he could have gotten that same list of names from Lydia, a Madrigal executive. He does just that, then promptly sets about having each one of them killed.