Did it cross your mind that Allen did it?

Yes! I was hoping someone else would have noticed this. It would have explained the "black-out" and the fact that you never see Leland as a psychopathic kid, but more of a depressed and overly empathetic one. Allen, on the other hand, seemed to be the definition of psychopath. Perhaps Allen was jealous of the attention Ryan was getting and was trying to get rid of the competition. Given Leland's naivete, it would make sense that his character would accept blame for something that he didn't do and just assume that, because everyone said so, he did it. There is a definite ambiguity throughout the movie that leaves it open to the interpretation of the viewer. In the end, though, it seemed that the writer had decided that, in fact, Leland did it as sort of a mercy killing.

Recently saw it and here were my thoughts.
Us of Leland interpretation
Allen (Chris Klein) could have been the killer.
His mother died a year ago when he moved in with the family.
He quit going to baseball practice yet didn't tell the family.
Now he is back slamming the ball against the garage door.
His reaction when girlfriends father tells at the reporters: "Talk to his parents, THEY are the ones who raised the monster. He looks rather angry."
After the stabbing of Leland he says:
"Now it's over..." Maybe he was tying up loose ends.

Leland tells his Mom "I think I made a mistake." He later tells a fellow inmate that he stabbed himself. He could have legitimately stabbed himself.

No. It was clear the Leland did it. Allen really was quite altruistic. He tried to warn Leland about Becky then rescued Becky later ranting at the drug dealer about how he was destroying Becky's family. Even in the end, while maybe he felt Leland was part of him losing Julie, his ultimate act was an illustration of his love for their family.

I don't love her.. She kicked me in the face!!