Kematu doesn't actually make any accusations about her betraying Hammerfell to the Dominion, only that she manipulates people.
She sold the city out to the Aldmeri Dominion. Were it not for her betrayal, Tanth could have held its ground in the war. The other noble houses discovered her betrayal and she fled. They want her brought back alive. The resistance against the Dominion is alive and well in Hammerfell, and they want justice.
DeanG is abolutely right in claiming that Tanth has been under Aldmer control for 20 years, but they could easily hunt her for that period of time - obviosuly she has esceaped them before.