If you tell a person they have no free will and they believe you, then you affect their moral behavior. If you tell a person they have free will and they believe you, then that person exercises free will. It is a self-fulfilling prophecy. If they think they have free will then they have it and use it.
Higher mammals have complicated brains and minds. Humans have billions of neurons and a mind to think. The mind is able to accept and reject info, and make decisions. Free will exists within a range. We have all witnessed individuals who have gone through tough and bad circumstances. Some of these individuals come out with sympathy for others, some come out hardened with very little sympathy for people in trouble. It seems each of these people has the free will in determining which result (sympathy or very little sympathy) to take.