All in her head
Is it possible that Brendan's character was just over-exaggerated by Miranda? I mean, the whole story was shown from her point of view, so she could've just been a paranoid that everything he did was aimed at her; with him still being 'in love' with her and trying to ruin her life.
When she first met Brendan with her sister, the first thing she said to him when she was alone was "Are you just doing this to get at me?" (or something along those lines). To me it seems like a person must be pretty self involved to think that, let alone ask it. Though by her asking that it shows that she was bothered by their relationship, and Brendan obviously saw this too and used it to his advantage (to entertain this notion she has).
There's no actual proof that he murdered anyone, just that he was obsessed with money (writing to his sister to ask about inheritance, selling Laura's house)and was a bit of a prick.
Whereas Miranda is the one that becomes obsessed with him, doing everything to find out about his past life, contact his sister, and find his new home. But it's still portrayed as if Brendan's 'the bad guy' because she's always viewed him in this was (probably 'cause he read her diary).