I've found the best way to short circuit a Karen is to agree with them and then go about whatever you were doing:

"Are you trying to kill me and my family?" -Yes

"Sir you need a mask. Here take one." -Oh thank you. take it and don't use it

"You are just a selfish asshole!" -Absolutely

The more positive and smiling you are the more it pisses them off. Try it. It's fun.

edit: Here's what I look like interacting with the Mask Greeter: https://hooktube.com/watch?v=NV8BlBKupHM