Okay, maybe don't kill them, but use an objective measure to scrutinize their claims to liking obscure bands. Put in any band to the obscurometer and get a % obscurity back. If you're registered as a user it can tell you how obscure your tastes are - this is where you send the hipsters to see whose is bigger. If they're cool enough they'll be into Lobster Toilet Death, which is the fifth most obscure. Coldplay, Radiohead and Red Hot Chili Peppers are the least obscure.
Of the Big 4 thrash bands, the obscurity in increasing order was Metallica, Megadeth, Slayer (almost same as Megadeth) and Anthrax. If you want to know the basis for the measurement go to the damn site.