Kreen had attempted to murder Tuf because he blames Moses' plagues upon Tuf, who has gained an interstellar reputation as an ecological engineer, and Tuf realizes that Moses' "plagues" (actually low-tech simulations easily imposed by sabotage upon the closed system of an arcology) offer him an opportunity for revenue.
After two such attacks, Tuf invites Moses aboard the Ark and shows him simulations of the increasingly horrible plagues that he could further inflict upon Moses and his followers.