I honestly believe Deb poisoned herself. The "evidence" linking Debs' accident to Hannah is so iffy to me. I was getting some manipulative vibes from Deb.

Deb totally framed Hannah. Deb is madly in love with Dexter, and she can't bear the thought of him spending the rest of his life with someone else, innocent or guilty. This seems to be one of those "if I can't have him, no one can" situations.

Well, first time i thought that Hanna poison Deb, but then i thought that everything was a set up orchestreted by Deb to ruin Hanna reputation in front of Dex...and then i got convinced that it was Hanna who really poisoned Deb.