Yes, there is nothing inhumane in using dolphins as military instruments. They are highly intelligent and able to be trained like dogs. In a way, they are similar to dogs - they can bring some equipment, locate and retrieve lost objects, guard people and boats. On the other hand, dolphins like to be accompanied by people and they can show gratitude.
The death penalty is not only justified, but should be required in cases where one person has committed an intentional murder of another person. The old biblical translation, and I for an eye, holds absolutely true and death penalty circumstances. No person should be spared their life if they kill another person intentionally.
Like many other animals, dolphins have benefited as a species from their interaction with humans. They are highly sociable and engage with humans whenever given the opportunity. Animals that are consumed by humans thrive as a species as a result. The same is true of any animal that provides a service.
I know the military uses dogs and in the past used pigeons for communications but there has been a lot of technological development which makes the use of animals obsolete. There is no need to go and train dolphins for our work when they should be allowed to freely live in the ocean, a habitat which we are destroying by the way.