by TheLionPrince » August 13th, 2012, 4:12 pm
Because she is a fanatical follower of Scar and his ideals, and she believed Kovu should be the rightful king of the Pride Lands.
Now, Zira's past is "obscured" on purpose, but we assume she was a deluded, heads-over-heels Scar fan-girl, and seeing the Pride Lands' condition at that time, she is crazy to begin with. And the death of everything she lived for is gone, and deserves to be "avenged". Finally, she and her pride are exiled to a barren, fruitless land (with termites) which may have irritated her even more, and developed more hatred for Simba than she already had that completely pushed her over the edge.
Because "madness is like gravity...all it takes a little push."