It wasn't clear to me either how he died. But I also didn't understand when the detective was reveling in his revenge and he heard his wife say thank you. If he went to kill the stalker but found him already dead, he didn't have time to rejoice before he was killed himself. I thought for a moment there Decim's head was going to explode.
like rawR said, the guy bled to death because of the fight with the stalker.
Also, he was not really a stalker, but rather a rapist..
And no, as I see it, the detective story was way back in the past (we see him younger). He avenged his wife, then started this psycho path of waiting for someone to make a despicable crime before killing them.
Some time after, one of his would-be target was the rapist of the younger sister. He watched the sister while she was being raped (because there needed to be a crime before he could avenge anyone in his own twisted mind), and then decided to pay a visit to the criminal (in order to kill him). He found him dead, and the brother, thinking the detective was the rapist's "friend" (the one his sister had told him about, who only watched), stabbed him too
We begin to see why the manager (blue haired little woman) is trying to change their way of judging (I think she is anyway~~) : right now, what the arbiters are doing is basically bringing the human's spirit to their breaking point and then judging them based on their actions, but as the detective said, everyone has its breaking point, only some are easier to break than others..
