Season one of The Magicians revolves around defeating "The Beast" or Martin. It's revealed that Jane is The Watcherwoman messing around with time and changing specific variables each time she sets a new "time loop".
So why not simply go back in time and kill Martin while he's in Fillory?
This is somewhat similar to "use a Time Turner, kill Tom Riddle" but that theory can be disregarded because the time turner only lasts one hour. In the Magicians on the other hand Jane can frolic around time as much as she pleases