Well, are we talking bukkit or vanilla? Because in vanilla the /kill command kills yourself no matter what you type in after it. In bukkit, I am less sure, but if it was anything like vanilla the command would be /kill @a to kill everyone.
In Vanilla if you type /kill <other player> it would kill them. In Bukkit, it does the same. Essentials is also the same, however you can add different parameters.
Mods had it for a short time at least. I think it was removed not too long after I was promoted... Not my fault <_<
kill a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,SophiaEmmaOliviaIsabellaEmilyChloeMadelynAbigailCharlotteAmeliaEllaKayleeAaliyah Don't forget Exasperaties HaileyHannahAddisonRileyAriaAriannaMackenzieLilaAdalynGraceBrooklynEllieAnnaIsabelleSophieScarlettaliLeahSarahNoraMilaElizabethKylieAudreyLucyMakaylaGabriellaVictoriaClaireSavannahPeytonMariaAlainaKenndyStellaLilianaAllisonKeiraAlyssaReaganMollyAlexandraVioletCharlieSadieRubyEvaElianaTaylorCalliePenelopaileyKaelynAlexisKatherine
This one is good. Probably myself as I can't die of anything else on Hellblock if not myself then KatoKillsMyGPA >.> You shall now die.