There's nothing that unites people like tragedy, and what could be more tragic than an untimely fatal exception error crashing your PC? Basically nothing. And that's why it's so effective to dress up as the blue screen of death and walk out on a convention floor or into a Halloween party. Everyone's reaction is the same: Oh, I know you, you bastard.
The blue screen of death costume can be as simple as printing "A problem has been detected and Windows has been shut down to prevent damage to your computer" on a blue sheet, or you can emphasize the "of death" part of the name and go full grim reaper. That makes for a nice contrast with the modern version of the BSOD, which comes with a frowny emoticon and has been shrunk down to a few lines of text rather than an intimidating screenful.
Either way, it's a costume anyone with a Windows PC can identify with. There are plenty of options for villain costumes, whether Darth Vader or the Joker or—most evil of all—a creeper from Minecraft, but a BSOD with a scythe made of RAM is always a solid choice.