According to the Peer Pressure page, the badge is awarded to those who "deleted [their] own post with 3 or more downvotes." I did this recently and didn't get the badge. The answer in question had 3 downvotes, but also had 4 upvotes. Either clarify the Peer Pressure badge to say "Deleted own post with a score of -3" or change the badge's implementation (and do the same for Disciplined, since I assume they're implemented the same way).
This has long been fixed. The current description is
Deleted own post with score of -3 or lower.