Okay, Theory Time!
I'm... preeetty convinced that Jack was partially possessed by somebody - Lacie or Alice (or Lacie possessing Alice who possessed Jack XD) - during the tragedy:
First, we have the picture of Alice with unconscious Jack who has his sword arm on a string, strongly implying manipulation.
Second, we have the fact that Jack was crying and terribly unhappy throughout the entire tragedy. He wasn't acting like himself (based on Gil's surprise at being slashed, Vince saying that everyone was acting weird because of Alice, how what Jack was saying out loud didn't always match with the agonized emotions he was showing). I mean really, there's a scene of a very miserable-looking Jack, apparently arguing with somebody in his head, saying "Stop it. I don't want to kill people!!"
Third, there is the scene where Jack first confronts Glen. Two odd things happen. First, Glen warns Gil not to come near them, because Jack isn't acting normal (as reinforced by Gil's surprise). Then, Glen attacks Jack. Now, Yen Press translated that last speech bubble of Glen's as "Jack... I must... I have no choice but to take your life--!!" but the actual Japanese volume says "Jack... I... [action particle towards] you--!!"
I dunno how Yen Press got all that out of the vagueness of the phrase (even though, based on the context of Glen charging Jack with drawn sword, we're expected to understand that the "action particle towards" is one of violence), but it still makes me wonder why Glen would have "no choice" but to kill Jack. I don't think Jack non-lethally slashing Gil would provoke that, and Glen was already on a killing spree, which is why Jack was confronting him with desperate questions in the first place. Maybe Glen thought Jack was contaminated and so Glen had "no choice." Maybe the whole city was.
Fourth, we got Glen standing around dead bodies muttering "Lacie." It's too soon to tell if Glen's words are of lament, longing, or blame, but it's clear that something related to Lacie was happening.
*~*~*
It seems like Jack was on a time limit (judging by Yura's reenactment of the massacre at here + 2 pages) to stop something by midnight (probably the sacrifice of Alice), and considering that the Clock Tower of Silence was frozen right before midnight, he barely made it.
Also, it seems that Glen might've done something way in the past that triggered a chain reaction leading to all this, and it weighs heavily on Glen's conscience and karma. Just... the way he talks to Leo during this scene: "Remember that you are the cause of all this. Remember, the fact that you brought him back to life does not mean you saved him. Don't forget that you did this!" And later Leo has internalized this too: "Everything that's happened is because of me!! Well, me and the Will of the Abyss."
While it's understandable why Leo feels this way, Glen's words are interesting. They imply that Glen might've done something similar. This could actually be the reason he wants the Will destroyed. Something he tried to do backfired. Maybe this could also explain why he's hoarding Alice clones. No basis for this yet, but I think it'll be revealed that the Baskervilles are conducting secret experiments to draw the Will into a human-ish body in order to destroy/claim it.
Things are really starting to make sense. I just wish it as a little longer... I wanna know how Jack got into the Barkervilles acquaintance and how he and Glen became such (at least apparently) good friends.