Prabhakaran would obviously want to finish off Karuna as soon as possible to establish his claim as the 'sole' representative of Tamils in Sri Lanka. The longer Karuna survives, the greater the loss of credibility for Prabhakaran.
Are Karuna's days numbered? Prabhakaran is still the master of terror, though, with his current bid for peace and the enormous international visibility his actions attract, extreme actions against a prominent target have become relatively difficult.