On first watch I thought the bar fight was WTF but watching again, they set it up brilliantly.
1) Pacifist fashionista dad guy confronts Karn and blames him for all the bad shit the Empire is doing. Karn is like WTF and fights him off with a rape choke.
2) Realizing he's hacked into unlimited power, Karn confronts Dedra and puts her in a rape choke. This gets him the answers he's looking for.
3) While Cassian is earning the "shittiest sniper in the galaxy" award for not taking any of the 87 clear shots he has at Dedra, Karn recognizes him as Fulcrum, blast doubles his ass and puts him in a rape choke. Of course it works and as Karn prepares to finish the job...
Cassian: Who da phuc are you?
4) Reeling from this sick burn and the reveal there are levels to this shit, Karn lowers his blaster too late because...
Pacifist fashionista dad guy: Fuck you and your fuckin' rape choke!!!