Gus was fortunate, but he was shrewdly playing the odds. The lab was the most logical place Lalo would g
Good points, but I can’t believe Lalo thinks they’d just kill this random whites women and not go back to her apartment to try to figure out who sent her. Between the car and Kim’s ID, there’s no reason to think Gus wouldn’t have found that address ASAP. So I can’t believe he ever intended on going back to Saul. If he wanted info from Saul, seemed he’d get it before sending Kim out.
I don't disagree with the first part but worst case scenario he never goes back to Saul's apartment. Leaving him alive at least means he has the option of questioning him at some point.
As far as why he didn't interrogate Saul, and then kill him, before Kim left you can look at it one of two ways:
1. Saul can't die because he's in "BB".
2. Lalo is impatient and wanted to set the plan in motion ASAP. He's been trying to prove the lab exists, and failing, for most of his character arc.
Fred.