So if I kill an enemy they should be allowed to shoot back with setback because of overlapping timing? I guess it really doesn't even matter if Han Solo shot first.
If you totally take them out as a result of your action, then no — they don’t get another action after yours and then wind up dying at the end of the round.
But if you don’t totally take them out, then yes — they get to take their action split seconds after you take yours, and while yours was resolved first, they do still get to take their action. Their action might be further modified by what you did as part of your action, but they still get to take it.
It’s not like you stop time for the entire field and only one guy gets to move or take any actions for an entire minute, and then the next guy gets to do all their movements and take all their actions, for their minute, and so on and so on.
This is why some people make the analogy to “Rocket Tag”. The party that gets to move first, if they can make their action big enough and effective enough, then the other party simply ceases to exist and never gets a chance to respond.
But if the other party is still standing and all you’ve done is piss them off, then they get to take their actions, and the timeframe of those actions is interleaved with the timeframe of your actions.
You may have been running into a doorway, fired some shots, and then ran out of the doorway, but during the time you were standing in the doorway and after you fired your shots, they got a chance to shoot back at you.