She must have known he was a psycho, yet appeared not to be afraid that he might want to silence her if she started having second thoughts. The offhand way she viewed the unexpectedly open door when she entered her house presaged her downfall.
shareWhile Josh was certainly a psycho to even come up with the idea to blow up the dam, Dena was, to be honest, a just as great psycho, if not even a greater psycho, which showed more than well throughout the whole movie
As for what then finally happened, Josh had very obviously no intentions to silence her when he first visited her, but wanted to merely talk to her, which was also made apparent in that scene. The only reason the situaton then escalated into violence, was because sheright out of the blue hit him in the head with whatever it was, when he was merely talking. Had she not attacked him, but instead calmly had the talk he had tried to initiate with her, nothing of what later happened would have occurred.
She was very obviously mentally unhinged already from the start, and it is not unusual that such people sooner or later create situations where they directly or indirectly get themselves killed, either with or without an interference with others (Suicide by cop, being an example).
Yeah, who knows what goes through the mind of a psycho? She might have had an unconscious desire to assuage her guilt by seeking death. She might even have deliberately become a threat to Josh as she knew he'd be after her sooner or later.
What is truly frightening is that there are thousands of people like Josh and Dena living amongst us in the general community, in all countries, who haven't been discovered yet and may well never be.