Killing Karen.
Wouldn't killing Karen at that point have been a terrible idea for Jimmy? I get why he had to kill Henry and possibly Karen, but he hadn't asked Henry to do the job in Florida yet. Hypothetically, lets say Karen doesn't have one of her two smart moments in the movie, and goes into the garage and "disappears". Henry knew where she was going, and when she wasn't back in a few hours, he would know what happened. Wouldn't he immediately grab his kids and go to the FBI, give all them up?
P.S. Her other smart moment was flushing all the cocaine during the raid, and hiding the gun. Despite what Henry said after he got out, they probably would have found it, and even if they wouldn't it was too risky to have that around.