> I'd be interested in pointers to prior art of things which started asinine, and wound up literally deadly.

I first heard 4chan mentioned in conversation around 2005 or so. It was a remarkably different cultural phenomenon then, as everyone has noted. I had been a regular reader of TOTSE's BBS long before 4chan emerged. TOTSE fits your description of a forum with humble beginnings in the 80s BBS counterculture, which ended up appealing exclusively to antisocial pursuits.