I HAVE AN ALIBI!!! Do you how hard it would be to travel 34 years into the future, stay there for a week and wait while someone gets poisoned, not burn her house down covering my butt, travel back to the past only to travel back to the future and end up being late to the crime scene???
Besides, I wouldn’t poison someone anyway. Too easily undone, plus I wouldn’t be there to make sure the “job” got done.
Besides… if I was the killer, there wouldn’t be a body, my fingerprints would be burned as well as her house while making it look like faulty wiring, and I’d set up a system to log her in and out periodically to put up the guise she was still alive…leaving nobody the wiser.