(i) Insulin being a hormone is produced in very little amounts in the body. Hence, a large number of animals need to be sacrificed for obtaining small quantities of insulin. This makes the cost of insulin very high. (Demand being many fold higher than supply).
(ii) Slaughtering of animals is also not ethical.
(iii) There is potential of immune response in humans against the administered insulin which is derived from animals.
(iv) There is possibility of slaughtered animals being infested with some infectious micro organism which may contaminate insulin.