I'm confused about what Zeno destroyed for killing Zamasu, did he destroy just the Earth, the entire Universe, or the 12 universes?
I'm assuming your referring to episode 67. Zeno said the following:
A world like this must be destroyed.
I believe it safe to assume that the earth was destroyed but not the entire universe. If Zeno did destroy the entire universe there wouldn't have been a way to use the time machine to go back and get him because time is a component of a universe. No universe, no time, no travel by time machine.
The above was my original answer to this question until I read chapter 18 of the manga and Whis said the following below.
Based off of this and prior knowledge we received in the anime that the King of All has destroyed universes before. I now believe it is safe to say that he actually destroyed the entire Universe in that timeline.
Previously I argued that if a Universe doesn't exist time can't exist either. And this is stepping away from anime for a bit but physics tells us that
Matter cannot be created or destroyed only transformed
So in summation after a little research I've come to a different conclusion.
Even if existence as we define it comes to an end. Time does not, we may lose the capacity to quantify or comphrehend it but time is everpresent.
I can honestly believe that this is a valid answer.
I would like to believe Zeno destroyed all the universes in order to start again because Zamasu killed all the gods—that is, the Kaioshins of other universes. Zeno, being the childlike being that he is, decided to reset rather than try to find a way to revive everyone that had died (which I suppose he would not do since it's "unnatural"). So, aside from the universe being corrupted by Zamasu, each universe was essentially without guidance.
When Goku and Trunks go to retrieve future Zeno, you could see that he wasn't floating in space space, but an odd, distorted, white/blank space. This doesn't confirm whether or not he destroyed other universes, but it would kinda be an explanation as to why Zeno is okay with traveling with Goku.
In the anime , Zeno states that a world like this should not exist and destroys the place. Since it states "world" and since the future timeline is different from the past timeline in DBS, I believe he destroyed the future timeline's earth, although I am just assuming this from the fact it says "world" in the anime.
I think what Zeno did was possibly destroy all matter in universe 7, or at least broke it down so it could eventually form or birth a whole new universe. So the way I see it, he did not destroy space time, he merely broke down all matter in that universe so it could start anew. As for why he left the rest of the universes behind to go to the present with Goku doesn't make sense to me. I guess one could argue that maybe he destroyed matter in all universes to reset everything for a new start, and would have to wait a very long time for matter to form into 12 new universes, similar to how our own universe took an estimated 15 billion years to get to where it is now from the Big Bang. So maybe he left that universe behind to go with Goku to the present because he knew he'd have nothing to do for billions of years. Just an idea...
I also think it's reasonable to assume he had to at least destroy all matter in universe 7 since Zamasu had begun trying to take it over. I would also think that Zamasu was trying to take over all universes since that was his plan originally, so maybe Zeno had a reason to destroy all matter in all universes since Zamasu was spreading so rapidly....