I do not support any of these people, this is just for educational/historical purposes.
Adam Lanza:
Blarvink / ChildRapistOhChrist(?) / CulturalPhilitstine / Fuckcomments / gayfortimk / GFTK / Kaynbred / KnaveSmig / Pedobear / queerforkimveer / Smiggles
Alek Minassian:
Xboxlightside
Alexandre Bissonnette:
blackbird101
Ali Sonboly:
Bernd / † NΞΘ† GҽR†™ / FSFlighterX / HunterZ / Neo / This is a Cyka Blyat /
Alyssa Bustamante:
alyssaheartsyou<3
Anders Breivik:
andrewbrei / Year2183
Andrew Blaze:
EGSAndrew / EGSworld / PioneersProd / PioneersProductions / Point2122 / PointProductions / WorthlessToaster
Arcan Cetin:
Arcan Cetin / Arcanmotherrussiavodkaandak47 / Aswellasdeeznuts / Letsgettobashingbuts / meowfraulein / The Russian / X xx Arcan xx X / xxArcan96xx
Artyom Anoufriev:
Anufriev92 / Corpse
Brenton Tarrant:
Commander Rockwell / fasffafasfaf / ferretlicker
Caleb Sharpe:
walrusmeat
Chris Harper-Mercer:
Ironcross45 / Lithium_Love
Connor Betts:
Flowers for Atomsk / iamthespookster
Darion Marcus Aguilar:
lovesthemota
David Ian Long:
doorkicker03
David Katz:
Bread / mr_sliced_bread / mrslicedbread / ravenschamp / ravenschamp2012 / StoicBulb / TREXHAVAHARTATAK
Devon Erickson:
brokencassettetape(?) / devonkillz
Dimitrios Pagourtzis:
_dimitrios_pagourtzis / Comma Kazi
Dylan Klebold:
<<-VoDkA->>
Dylann Roof:
AryanBlood88(?) / derekjeter88 / LilAryan
Elliot Rodger:
Elliot Rodger / ElliotRodger / ElliotR1 / GeneralT0j0(?) / MiiRikSongPlayer(?) / Ramsaybolton(?) / The Purifier(?) / TheWoWGenius(?) / Valtharion(?) / Varodan123(?)
Eric Harris:
REB / Rebdomine / rebldomaker / rebdoomer / rebthejuvey
Esteban Santiago:
estebansantiago64 / naota33(?)
Gavin Eugene Long:
ConvosWithCosmo / Cosmo / Cosmo Setepenra
George Sodini:
MOSB46PGH
James Holmes:
classicjimbo / sherlockbond
Jared Lee Loughner:
classitup10 / starhitshnaz
Jarrod Ramos:
EricHartleyFrnd
Jaylen Fryberg:
frybergj / jaylen.fryberg
Jeff Weise:
blades11 / Jeff / nativenazi / Regret / todesengel / weise
John Russell Houser:
jrustyhouser
Joshua Smith:
Bobby Lemon / bobbylemon / BobbyLemon15 / JoshuaM6964491 / Joshybo44427233 / Mr. Smithy / sandman2
(NOTE: Not a killer, mainly a lolcow - Also im not including his telegram username so people wont add him)
Kimveer Gill:
fatality666
Leighton Labute:
Dollyflesh / Fleshdolly / MiseryMagic
Lindsay Souvannarath:
cockswastika / HackerOnHacker / Heretics on Holiday / Hyperborea Odyssea Hackeron / Lindsay.kanittha / snoopyfemme
(NOTE: Would-be mass shooter. Was arrested before she was able to carry out her attack)
Matti Saari:
Wumpscut86
Mohammed Alshamrani:
M7MD_SHAMRANI(?)
Nasim Najafi Aghdam:
Nasim Handmades / Nasim Wonder1 / Nasimesabz1
Nikita Lytkin:
fuckinnefor / Nikita Lytkin / Расчленённая ПугачОва
Nikolas Cruz:
crazynikolas_new / cruz_nikolas / nikarov694 / nikolas cruz
Patrick Crusius:
Outsider609 / PatrickCrusius(?)
Pekka-Eric Auvinen:
Naturalselector89 / Sturmgeist89
Randall Shepherd:
Evil_Toe
(NOTE: Would-be mass shooter. Was arrested before he was able to carry out his attack)
Robert Bowers:
onedingo
Salvador Ramos:
disrvpts / revcnge / salv8dor / salv8dorr / salv8dor_ / TheBiggestOpp
Santino Legan:
santino_williamlegan
Sebastian Bosse:
ResistantX
Seung-Hui Cho:
blazers5505
Tim Kretschmer:
JawsPredator1
Trey Eric Sessler:
LensCapProductions
Vladislav Roslyakov:
Big Slims / dablvanish
William Atchison:
Adam Lanza / AlGore / AMOG / Augustus Freeman / Baldo McFaggot / barajara / blades11 / billisbill / Blockdude / Bob 2.0 / ChoSeungHui / Cho Seung Hui 4.0 / Demetrius Alcala / DemetriusXVI / Drake Goodwin / El Digato / Eldigato / ElDigato / FIBAgent / FuckYou / Future Mass Shooter / Ganon / GNAA Initiate / Lance Stone / MrApophis / phantommastah / SatanicDruggie / Sephiroth / Sturmgeist88 / TheOrangeBlock / v0002 / Vance Stone
Based on This Reddit post , but I added a few ..
I'm aware i'm probably missing some, if you know any please let me know