I saw this typical complaint on Twitter, to which Stacy McCain replied:

@adaminoakland James Holmes was (is) a psychotic who identified with the "Joker" character in Batman movies. Kinda different.

— Robert Stacy McCain (@rsmccain) April 28, 2013

Different, but also the same: when a psychotic kills someone, because they are psychotic, we're told not to stigmatize psychotics:

• Stop stigmatizing mental illness, By Gene Lyons, January 4, 2013
• Gun debate stigmatizes the mentally ill: As I See It, By Brice H. Smith, April 17, 2013

And this complaint, from  "mental health advocacy group"which has the excuse that the people who wrote it are crazy themselves, the other ones writing about stigma are technicall sane:

• Explain to cartoonist Clay Bennett why comic is stigmatizing | PEERS - Promoting Mental Health, Empowerment, and Social Inclusion

They've complaining about this cartoon:

 

An extreme example of not stigmatizing: Vince Weigang Li is the Chinese immigrant who killed a man named Tim McLean on an bus trip in Canada. Weigang Li "stabbed, gutted and beheaded" McLean, and also "cannibalized the victim and pocketed his nose, lips and ear."

In discussing whether the cannibal killer should be allowed to leave his mental hospital on passes, McLean 's mother used the phrase "“I’m not stigmatizing mental issues."[Cannibal wins right to leave mental hospital,  by Joe Borlik, MyFox8.comMay 27, 2012,

When Muslims kill, it's "Don't stigmatize Muslims."

When immigrants kill, it's "Don't stigmatize immigrants."

And when dangerous wackos who should have been locked up kill, it's "Don't stigmatize the mentally ill."

Who can be stigmatized?

• Sarah Palin:Poisonous Politics
• Jared Taylor:Never Let A Tragedy Go To Waste: Gabrielle Giffords, Jared Taylor And The Agenda Of The Left
• White males:Jezebel: "Have You Noticed That White Dudes Keep Mass Murdering People?" No, We've Noticed You Promoting It
• Guns themselves, and American gun rights: Gun Control—Like Immigration Law, Enforced Only Against Those Who Obey It

The difference between the first list, of Muslims, mass murdering immigrants, and maniacs, and the second list regular, of  Americans and inanimate objects, is that the people on the first list are guilty, and on the second list, not guilty.