He was supposedly looking for a world where he belongs with his powers. At least that's what he implied in the last issue. The real question is, why interfere in their history? Why remove the JSA? Unless he was trying to prevent super-heroes from coming about in this world same as he did his own given that it ended so badly?
Assassinate Putin!
From Geoff Johns' panel at NYCC:
-Two covers of the book are shown, one is a Legion flight ring and the other is Manhattan causing all the heroes to vanish
-Geoff thanks fans for the positive reception
-Gary Frank is the best artist working today
-Two Covers are shown for Issue Ten, one featuring Nathaniel Dusk and the other showing Carver Colman reading a copy of All-Star Comics that has Doctor Manhattan on them.
https://www.newsarama.com/42192-nycc...spotlight.html
(Apologies if what Miles posted was either of these, it's not loading for me)
Doomsday Clock #9:
Doomsday Clock #10, presumably the variant:
The significance of the former to Superman is obviously pretty apparent. The latter is clearly important in the implications to the DCU all on its own...but Superman was on the team at that time if we're going full meta with it the way this appears to, albeit in a reservist capacity. Given Manhattan's interest, that hardly feels unimportant.
Buh-bye
I am surprised that they haven't mention nor shown how Doctor Manhattan messed with Superman's past yet.