Jane was gonna snitch breh. Talking about she would expose Walt if he didn't pay up. Then even after he dropped the money off she was like
"I might keep quiet" like it's some shyt to joke about.
Then on top of all that she was a drug addict. Like Walt said, with all that money Jane and Jesse would've been dead quick fast. He didn't always show it but Walt cared about Jesse.
As far as the fly, it was Walt's conscience. That's why at the end of the ep Walt told Jesse he was sorry about Jane....that's also why the fly reappeared in "Gliding Over All", directly after Walt killed Mike.
Fred.