Clue #1: Mr. Moore and Mrs. Kelley - Ashton Kutcher married Demi Moore, so you could consider him to be “Mr. Moore”. Katherine Heigl married Josh Kelley, so you could consider her to be “Mrs. Kelley”. These two starred in a 2010 movie called Killers.
Clue #2: SARS-CoV-2 and HIV - A pair of viruses that have killed many people, thus they are killers.
Clue #3: Drinking the Kool-Aid - In the infamous Jonestown incident, most of the population killed themselves by drinking poisoned Kool-Aid (apparently it was actually Flavor-Aid, but whatever), and this led to the phrase “drinking the Kool-Aid” meaning being brainwashed or going along with someone else’s bad ideas without questioning them. The people who literally drank the Kool-Aid did it to kill themselves, so they were killers. Also, Kool-Aid Man’s catch phrase was “Oh Yeah!”, which Paul “The Manno” Di’anno squeals several times during “Killers”.
Clue #4: The Mercenary and The Assassin - While these are both Maiden songs, they are also occupations that involve killing, thus these sorts of people would be killers.