From what I've heard, it has yet to be revealed. Some people believe it was Glen or Vincent but nobody knows yet. I personally think it was Vincent but I don't know.

It hasn't been told yet, I also think it might have been Vincent since she looked as if she had been cut by sciccors

I have to say it was Vince since with the knowledge we gained from the recent chapers we can say that Gilbert was in the ritual to uptain Raven with Glen (so neither of those two could have done it) Also if I remember correctly Cheshire had his eyes ripped out with sciccors (Vinces favorite weapon)So the obvious person would be Vince (But this story is full of twists so you can never know)

you may think its wired but I think that the one who killed Alice is JACK...

I think we're supposed to be led to believe it was Vincent, but as for the mangaka's story telling style, when it's finally revealed who the murderer is it'll probably be someone totally unexpected... >>; Actually, now that I consider it, it /doesn't/ sound totally improbable that it was Jack o.o

The murderer is portrayed as Vincent, but with the way the story is being told it seems to me that it could have been Gilbert. But with the way Jack acted in the most recent chapter It could have been him or Glen! Who the hell knows what's going on in Jun's mind?

I believe it was Jack. In some of the later chapters, what I got, is that he's not the man he appears to be. I don't think they'd make the person who killed her so obvious, like portraying it as Vincent.
But as Diamante405 said, who the hell knows?

I really would'nt be surprised if she commited suicide using the scissors, but like everyone says lol who the hell knows lol.

Honestly at this point I have to say it was Jack. He has turned into someone I never even knew he was. Plus I have always felt something strange about him since the first time I have seen him. So before I move on please tell me I am not the only one. Anyway, back to my proof. Some people may argue that there were a pair of scissors at the crime scene (Vincent's favorite item ever) but one, If he had stabed her with them then wouldn't there have been more blood on them then there was? And two for the matter, it was Jack who shows Oz that so how do we know that's how it all happaned? Everything else he showed us towards the beginning is a absolute lie so why should this be a exception? Anyway, Glen is also cut out of being the culprit because he was killed by Jack soon after Gil was slashed in the head by being beheaded plus he loved Alice like she was his daughter so why would he do that? Vincent is cut off because for one thing, it said it JACKS journal that he doesn't like Alice so why should we trust those words? Plus he was off being the hero and rescueing Gilbert, and opening the gate to the Abyss not knowing what would happan............ Anyway, Gil couldn't have done it for obvious reasons that I will not point out cause if you don't know your a idiot. So who else is there that could have done it? Plus the fact that Jack had brought Oz to a memory of Alice with the Oz doll towards the end of retrace 70 says something he is going to show Oz, and he said he was going to tell Oz the answers he seeks. And when Alice started to choke him in retrace 1 she said she will never forgive him and it says later on that Oz in the past promised he would protect her from anyone who was going to harm her. So who wouldn't Oz disobey in the past? His conracter Jack. So either Jack ordered him to stand down and not help her or ordered him to kil her himself. Plus the fact he was the one causing all the deaths of the tragedy doesn't help him look good either. So over all, it was most likely Jack who killed Alice, just think about it and hopefully you will see my reasoning.

Alice actually killed herself. She thought that Jack was going to use her blood to turn Oz into B-Rabbit. Seeing that she loved Oz she killed herself to save him (at least she thought she was).

I believe that it was Vincent. Along with all of the other comments here that believe this, Vincent has always been portrayed as the bad guy in the series, and you see him holding a pair of scissors during the memory of the tradegy of Sabrie and he also killed alice's cat Cheshire. And then Cheshire became a chain and well, you know. But It might have also been suicide. But I really do think it was Vincent.

Alice killed herself Jack was going to used Alice or Alyss' powers to destroy the world.

Gilbert, could not have killed Alice because... I really don't want to say it but he didn't.

Alice called herself. Because of Jack. But in the manga Vincent said it was all Alyss' fault...

And I personally think Vincent cut out Cheshire's eyes because Gilbert hates cats.
And because he resents Alice for making Gil 'react that way'. And for making Jack worry.

But yea Alice killed herself. I never really new why though...