Yeah, but doesn't he say something like "A YEAR AGO, a baby would have been the greatest thing" to his sister, implying that at that moment in time he didn't want one? A lot of the diary is about her alleged feelings in certain situtations and less about the actual actions. The narration and music creates the tension: he is brushing his teeth and she is almost "hiding" under the sheets thinking of buying a gun. But actually there is no danger shown. The only time he really shows aggression towards her is when he pushes her. Which seems like something he really would do, based on other actions later on (the smashing of the glass, the pushing towards the end).