Okay, so I was just in a game with KeredNomrah69 and RequixEclipse. It turns out that Liam's theory on how the Red Insta-kill is obtained was false. I tried it three times playing as misty and got nothing. The strange thing is that KeredNomrah69 who was playing as Marlton got the green puff of smoke after killing zombies on Insta-kill and for the two Insta-kills after I had the Red Insta-kill. On the first Red Insta-kill I didn't knife at all and I still had it for the second one we got. On the second Red Insta-kill I knifed and lost it. My theory is that when you get the Red Insta-kill you can't knife or you'll lose it. Keep testing everyone we still are not 100% sure how to obtain the Red Insta-kill and keep it.