Magus on a Star Trek wrote:
I’m quite bugged by Dexter’s treatment of Hannah and his statement that “If there had been any other way, I would have found it.” He didn’t even try to look for another way. Wouldn’t it have made more sense to use the pen for blackmail than to lock her up? “I have the pen. Anything happens to Deb or me and it goes to the police.”
If he tried that, he almost certainly would have been the NEXT one on her list. Hannah might truly love Dexter, but she's still got some major
issues and it probably wouldn't take much for her to go into self-preservation mode and start offing those around her.
As Dexter says, you can never fully trust a poisoner.
And I think people are reaching when it comes to the water bottle thing. The only reason Hannah froze before admitting the truth was for dramatic effect. And no matter how much you over analyze the events, it just doesn't make any sense for her to accept the blame if she had nothing to do with it.