How is Lord Kultin deceitful in the novel, A Vow of Glory?

Lord Kultin, the leader of Gareth’s private, paid soldiers, stands behind Gareth when Gareth tells the people of King’s Court that they must go after Gwen and kill her. After the soldiers who have chosen to separate from Gareth are gone, however, Kultin creates a plan to kill Gareth and take over as ruler.