originally, the writers had planned on Quelling bringing a gun that day and threaten the firm because of what Harvey did. But Korsh and the writers decided it would be too much emotional drama for the show and decided to do this scene instead. Hence, the gun reference Harvey said in the scene
"Well, then you better look over your shoulder every day for the rest of your natural born life."
"I wanna know what’s going on."
"I told you what’s going on, Harvey and I are taking care of a situation."
"That’s a bullshit answer, and you know it."
see, I’ve been a bad, bad, bad, bad man
and I’m in deep, yes I am
I found a brand new love for this man
and I can’t wait till you see