Some of the first glimpses we get of Will of the Abyss (Alice), is not necessarily her true nature.
Especially when we see the picture in a more recent chapter where Alice (Will of the Abyss) is laying on the ground seeming to be asleep. When she opens her eyes, though, she appears to be very weary. I believe instead of being mentally unstable, she is actually still in the mentality of a young and sheltered child. She doesn’t see much of the real world, especially when her twin lives in a tower isolated from the rest of the Baskervilles. The following is the picture that I mentioned.
I don’t care that a lot of people will disagree with me on this…but I feel that the Alice twins were victims from their very conception.
Their mother was a child of misfortune, the sister of the new Glen Baskerville, Oswald. Revis took advantage of the fact she was going to be sent to the Abyss anyways to use her for his own sick experiment. After impregnating her (and we all know how that would have been done back then >< ), he leaves her to be killed without telling Oswald what he had done to her. When Lacie’s body is ripped apart in the Abyss, the twins she was carrying within her are nurtured by the Abyss and then aged several years before one is sent back, while the other becomes the host for the core of the Abyss, thereby becoming the Intention or Will of the Abyss.
Jack loved Lacie Baskerville, as we all know very well by this point. She changed his outlook on life and gave him a reason to live. But then he becomes best friends with her brother until he becomes Glen and is forced to kill her. Jack was manipulated by Revis too, as he was the one to tell Jack about Alice and about the chains that surrounded the world keeping it from the Abyss.
Honestly, the one responsible for the Tragedy was not actually Jack in a sense. His love and grief for Lacie were manipulated.
Photo sources: http://fc08.deviantart.net/fs71/f/2012/070/2/1/will_of_abyss_rabbit_render_by_xyanderegirl-d4sfzg6.png, http://crazy-cake.blogspot.com/2009/08/pandora-hearts-episode-7.html, http://static1.minitokyo.net/view/28/31/571578.jpg, http://img188.imageshack.us/img188/5688/pandorahearts223.png, http://media.photobucket.com/image/pandora%20hearts%20will%20of%20the%20abyss/kitari-pyon/Pandora_Hearts_07_02.png