The group is gone now. A community guidelines violation, or does a certain someone no longer hate you? Maybe he made it private.
I was hoping he would leave the group. That usually makes the next member who was in it the longest the admin. Happened to me a couple of times. Of course, I quit those groups because I didn't want to be an admin.
In fact, as far as I can tell, there is no way to delete a group. I've been meaning to get rid of one of mine that failed, but the only thing you can do is quit it. If you are the last member and quit the group, it closes it.