There's no need for them to, in the manga Trunks already said it a while ago that the Kaio-shin in his world was killed by Dabura during the Buu incident.
Assuming that Black is the new God of Destruction from that world, Beerus, the only person that can easily stand a chance against them not being able to fight him would all conveniently play out as one of the most important rules states that 2 God of Destruction must never fight each other. And even then, yes they WOULD kill him if he had been alive. Zamasu is almost certainly going to kill Zamasu next week as it is.
Quote:If anything it seems Zamasu and Black are only targeting humans....
Not exactly, They're targeting ALL beings that aren't gods/below them. Black himself said that he had already destroyed countless worlds before reaching earth. One of the major problemes with the subbing community is that they keep translating Ningen as Human. Normally this is right beacuse Ningen NORMALLY literally means human. However, in Toriyama's world the word Ningen is specifically used in general for basically "mortals" that aren't godly beings. Humans, Saiyains, those caveman aliens, etc are all "ningens" as far as the contect of the series goes.
As to Zen-Ou, he shouldn't have any need to interfere and probably doesn't care. At least not until Goku accidentally pushes that button. There were also 5 green rings of time that were created whever someones screws up time and creates an alternate world/timeline, so who knows how that's going to play out.
Albeit, contrary to what western fandom seems to think, Trunks' world is the true original timeline that came from the Dragonball series. Goku's/main Super timeline is the one that's just an alternate future that gets the focus.