Voices and accents are one of the easiest ways to distinguish NPCs, especially villains. I encourage everyone to indulge in epiglottal impersonations. And anyway, DnD is more fun with ridiculous voices.
One tip: pay attention to the pauses and the timing. It's easy to overlook, but it adds a lot.
Chigurh from No Country For Old Men
He's a rambling, unrepentant murderer. He's my new favorite.
In case you want a psychotic bad guy without a recognizable precedent.
Inspiration for Heath Ledger's Joker
In case you want a bad guy who's on a lot of drugs.
Emperor Zombie in The Incredible Screw-On Head
Invented by Mike Mignola, the Hellboy Guy
In case you want a clever, playful bad guy. [skip to about 8:04]
Tony in Scarface
In case you want an intense little thug as a bad guy.
Tetsuo in Akira
I hope you've seen Akira. There's a lot of visual, music, and ideas you can cull.
In case you want a sweaty, persecution-complex kid as a bad guy.
Gotta get some non-fiction in here.
In case you want an insane egomaniac for your next bad guy. Or a hobo.
Holy shit, Manson would be a great evil hobo-wizard. With crazy groupies.
[skip to about 2:25 if you want to see him wig the fuck out]
Obligatory evil mastermind voice.
In case you want a debonair, psychological bad guy.
Obligatory basso monster voice. There's actually a lot of good examples of stereotyped voices in this video.
In case you want a rumbly, labored monster voice. Pretend you're a talking dump truck. Or a sewer.