Jenna thought she was going to be able to just coast into the final four and probably felt her alliance with Heidi would take the both of them into the final two.
She showed with the Deena situation that she does not like anyone to deviate from the agreed plan of action.
If someone makes a pact with her she wants them to keep it.
It would be interestign to see what she would do if faced with a dilemma that called for her to break her word.
I have a feeling she'd do it.