In _Legends of Tomorrow_, s2e7, named [_Invasion!_](http://arrow.wikia.com/wiki/Invasion!_(DC%27s_Legends_of_Tomorrow)) after vibing to the spaceship of the Dominators and talking with one of them, Cisco somewhat lamented in the following [conversation](http://transcripts.foreverdreaming.org/viewtopic.php?f=557&t=30070&sid=7bcdfeb02a822a67facbfb714a9b2e84) with Nate:

> What if, in using our powers to make the world better, we actually end up destroying it? 
>
> Dude, I get it. 
>
> Barry screwed up. Yeah, __so did we__. 
>
> Back in 1951. 
>
>  __We messed with time when we freed the Dominator__. 
>
> Who's now coming to kill us.


So, Cisco thought he messed up the timeline by saving the dominator back in 51 from the hands of Agent Smith as they were now coming to kill them.

I could not comprehend this scenario.

The Dominators came in 50s to assess the extent of threat from the metahumans due to JSA and for some reasons departed.


They would eventually return to assess the status of the threat with the rise of metas after the particle-accelerator explosion.

The point is that their return has nothing to do with the event of Cisco and Felicity & Legends saving the Dominator back in 51.

So, how did Cisco by saving the dominator mess up the time? Why did he think his action was responsible for their return?

Won't the Dominators still come back due to the recent rise of metas irrespective of whether or not Cisco and co. saved a Dominator or not due to the rise of metas?

I'm, in a word, failing to see the reason of Cisco worrying and holding him responsible for their return to "_kill_" them.