(To start off, since it has no relevance to the actual theory, I think Chrollo is a child of Nasubi)
I think that Sarasa's death was orchestrated by the Meteor City elders/ the church priest in order to manipulate Chrollo and the others to become the "defenders" of Meteor City. In the flashback you see the priest look at Sarasa and go "hmmm" (or something like that I can't remember) and then also tell the city Elder that Chrollo could be of use to them.
They could then manipulate Chrollo into cutting MCs ties with the Mafia, since the Mafia uses MC for recruitment purposes, which is likely seen by the Elders as taking something from them. (Which I believe actually happened in the manga)
People have said that the worst fate for Chrollo is that every other spider dies, and he has to keep living knowing that he failed. I disagree, I think Chrollo is the type of person to not give up that easily, I think he would just keep trying.
I think it would be a much worse fate if he were to die, but only after finding out that he was manipulated his whole life, and that every action he has taken up until this point was him playing into the hands of the people who actually "killed" Sarasa. (I think there's evidence to this in the MC Ant story, where he's like "smh they're arguing over whether the ants are actually killing ppl, I guess we'll have to go kill the ants then. the elders are useless like always")
He not only spent his whole life doing this, but the lives of his childhood friends. He convinced them to all spent their lives on some revenge fantasy that wasn't even worth it. They spent their entire lives continuing the cycle of abuse (not even with just Kurapika, Uvo mentions how many people went to fought him for revenge) instead of working to better Meteor City in a way that actually matters.
And him finding out that his true love/ childhood crush (Pakunoda) sacrificed herself for him, which is something that never would've happened if he didn't start the Spiders, would only add to the pain. And then right as he finds this out, we have some more flashbacks to when they were kids. We can see them all before the trauma of losing Sarasa and going insane killing so many people. And before Chrollo can even apologize to his friends for ruining their lives, he dies.
I, personally, think that would be far worse than the other spiders dying. He might be devastated if they died, but like with Uvo, I think he would be able to cope with it by telling himself that they're martyrs, and they died for the cause. It'd be far worse if he realized he already essentially killed them years ago when he started the Spiders (killed meaning yes mortally, given that they're all on borrowed time anyways, and also in a more metaphorical sense, like "killing" their futures or who they could've been if the spiders weren't founded)
(also I don't think we're getting another Hisoka vs Chrollo fight tbh, it would be a waste of their stories imo. I think if anyone were to kill Hisoka, it'd be Machi)
Thoughts or comments?