Utilitarianism, as one of the consequential theories, tells us that net balance of the happiness over unhappiness is the only thing matters. As long as an action creates the best consequence, the action is moral. However, Kant thinks that the intention of an action determines if the action is moral or not. He requires us to take steps and figure out if the action can pass the universal maxim. Utilitarian cares about the consequence of the action, and Kant thinks that the intention of an action is more important.
They also have different attitudes about how animals and human beings should be treated. I think both of them have problems when they try to apply their theories between humans and animals.
Kant clearly separates rational beings from animals, babies and cognitive imperative people. However, Kant runs into a problem that he fails to explain why we should not torture wild animals for fun. Even though he gives us reason that a person who tortures animals has a tendency to torture human beings, I am still not convinced by the reason he gives us. There are some evidences shows that serial killers start from killing animals. Regardless of their psychological changes, it is hard for me to see the philosophical reasons of why we should not torture wild animals based on Kant’s view. I do not think Kant provides a clear reason of that. (only based on reading materials I read.) Maybe he can say something such as if we torture wild animals, it is a disrespect for ourselves. If that is his explanation, then our reasons for not torturing animals is not because we respect lives, but because we do not want to disrespect ourselves. I think it contradicts his view of humanity. (Maybe I misunderstand his argument?)
Utilitarians, however, treat every species impartially on the earth. They think that we should count every species’ wellbeing as the same. However, I get a little bit confused about how they can measure the wellbeing of animals. I do admit that human beings are really good at predicting the future. (most of the times.) We also intuitively know that what kind of actions would maximize human’s wellbeing. However, I think we would have trouble measuring animals’ wellbeing. We can be wrong when we try to use our ways to measure animals’ wellbeing. For example, we might think that adopting a homeless dog from the cold, desperate street can maximize the dog’s wellbeing. We think that we can provide the dog a better living condition and feed him better food. The dog would be happier living with us instead of living on the street. However, what if the dog just want to stay with other dogs on the street and be free all the time? How can we read their minds and know if they are happy or not? We can be wrong to measure and predict the consequences of our actions on nonhuman animals. I do not know if it is a problem for utilitarians.