Has been stated officially that it was Minato who gave BEE the title of shinobi killer? I seem to want to think so.
Yes, Minato did in fact call Bee a “Shinobi Killer” in Naruto Chapter 543. This occurred during Minato’s battle with B and the 4th Raikage.
The two from this encounter Minato would come to have much respect for B and his abilities. Minato even stated that B acted as a true shinobi killer, thus giving him the title of "Killer"