We still don't know what was the role of future Syd. Was she a delusion or was she real?

I've only just watched it so, this is a late reply I know. "Future-syd" was Farouk in disguise as is hinted at in one of the cartoony/CGI bits of the fight where Farouk in monster form rips out of Syds chest. David was never going to end the world, that was a lie, and David in that "future" didn't kill Farouk, that was a lie - Farouk essentially tricked past David to help him not only find his body, but turn everyone against David. Farouk wanted David dead or disabled on medication, and it would just be icing on the cake that David's friends would be the ones to keep him that way.

Farouk didn't count on David being as powerful as he is and escaping of course.

Side alt theory: Farouk isn't trying to kill/disable David, but turn him into Legion (whatever that is, I haven't read the comics) for some clever manipulative reason.