I'm certain that she was the high priestess to some nihilistic doomsday death cult that rode the Chuckles and Kimmy sideshow to fulfil some tangential prophecy that required a universal extinction event to strike just so, just then.
It's even possible that she was a pan dimensional being that had been setting things up just so for hundreds of years to create this exact situation: an appendix universe set adrift from the main timeline.
If Tessa was a young fit human being, and her boyfriend, an old man, told her that he wanted to destroy the universe and unmake most of her life, the only sane response is to hit him in the head with an ash tray again and again until he is dead.
Imagine your partner admitted that they are a Russian spy and they need your help to destroy the parliament House in Adelaide?
Do you agree to help, or pick up a heavy ash tray?
It doesn't make sense.
Unless she was a sexbot?
I'd buy that for a dollar.