I think you were misled by the editing: Because she lost, the editors didn't show us all of Chrissy's interactions with the others but, as she stated in her speach, she got to know everyone on a personal basis. Personally, I'd say the word "determined" applied much more than "vengeful".

When Chrissy targeted Ashley, Ben had played his 92nd hidden idol so she couldn't get him out.

And even if it was possible (it never is) exactly what would have following Ben 100% of the time accomplished? Once he'd found it, they couldn't steal it from him.

And like I said, they are simply not allowed to follow someone 100% of the time. When production asked him for a confessional they had to let him go alone. No one knows exactly how long those confessionals last so, once he was done, Ben could still have spent time alone finding his 200th idol.

Devon impressed me but HE was the one that missed the real chance of getting rid of Ben. Ashley told him it would be good to vote for the Marine instead of Joe and, at that TC, Ben was vulnerable.