In Chapter 111, Gon asks Chrollo "How can you kill people who have nothing to do with you?". Chrollo initially answers that "It might be BECAUSE they have nothing to do with me", but he also says that it's hard to answer when he's asked like that. Over 20 years later, we finally know why he does that. The Phantom Troupe that we know of (criminals instead of voice actors) were formed after experiencing the loss of their friend Sarasa, who was murdered, and the 11 year old Chrollo will never forgive those murderers for what they did. Though there might be more to it, as there's also a note that Chrollo read and refused to tell anyone what the note said, so even us readers are kept in the dark about it (all we know is that the child abductors killed Sarasa for fun, as they had already met their quota). And of course, the Troupe all chose Chrollo to be their leader.
And now we know the backstory of the Phantom Troupe (well, I assume Formation Part 3 is the final chapter of this flashback, and if it is, I enjoyed this flashback a lot). And this was almost 9 years after I watched the Yorknew City arc for the first time in Hunter x Hunter (1999).