I went to Hell and went as far as I could go - until I ran out of streets signs and had to turn back. I didn't get the badge. I've now filed a bug report - thanks for the responses.
I went to do the Completist Badge for Naraka and had the unfortunate event of levelling in Hell because of it (you don't get anything if you level up in Hell).
I died and went through hell, didn't get a badge then. It showed up the next time I log in, during a new in-game day. Maybe it'll work if you relog on a new day, too?