Aaron Hotchner, badass, tied for the lead with 4.25 (The Tribe, A Real Rain, Legacy (split with Morgan, Prentiss, and the CotW), and Catching Out--and then he beat George Foyet to death with his bare hands in 100.). (I just realized I've been listing Psychodrama, and that UNSUB survived.)
Derek Morgan tied for first at 4.25. (Charm & Harm (split with Elle), Lessons Learned, Open Season (split with Prentiss), Legacy (split with Hotch, Prentiss, and the CotW), and Minimal Loss (three in the center of mass. Now, "Outfoxed," more or less in extremis. Do not mess with that man. Is that the first time they have snuffed a female UNSUB?).
Future vigilante killer Elle Greenaway: slips to third--but hangs ON to third--despite having left the show in 2x05, with 2.5 (Extreme Aggressor, Charm & Harm (split with Morgan), Aftermath)
The mild-mannered but deadly Dr. Spencer Reid, fourth place with at 2 (LDSK; Revelations)
Emily Prentiss, fifth with either 1.75 or 1.25, depending on whether one of those offscreen gunshots in Open Season was actually hers, or if all three were Morgan's. I award her the kill because of her reaction on the plane afterward. The .25 is the other quarter of Charles Holcomb in Legacy, who went down like Frankie. The 1 is the suicide-by-cop in "Lo-Fi."
JJ: 1, in Penelope. And RIGHT THROUGH THE FBI SEAL. Hello Symbolism Much? Also, three dogs and my eternal respect. She is 1:1 bullets:kills, at this juncture.
Rossi: in second-to-last place with 1, in his very first episode (About Face). Having established that he is a badass, he has never badassed since. ("Welcome to the Enterprise. Would you like to beat up my Klingon?")
Garcia still doesn't kill people.
Gideon has never discharged his weapon. However, he has beaten somebody all to hell with a clubbed shotgun.
And of course, various shootings perpetrated by unsubs, victims, the families of unsubs, cops of the week, snipers, random bystanders, cultists, lovelorn teenagers, and Frank Miller's ego....