Why do the excutions take place? What message do these events send to the animals about their role in a future society?
Napoleon starts executing animals to create an environment of fear to keep any further thoughts of rebellion or revolt from the minds of the animals. The hens who were responsible for the revolt after they were required to surrender their eggs are slaughtered, the porkers who admit to having contact with Snowball are also killed. Napoleon is establishing a state of fear and suppression of freedom in order to maintain total control over the farm.
The executions signify a new day on the farm where all the animals are not equal, the pigs and the dogs, Napoleon's bodyguards and killers, are more equal than the other animals.
All animals should be warned, in the future, their role in society will be to work hard, ask no questions, be satisfied with your meager ration of food, do not complain about the smaller portion of food, never question anything that Napoleon does or says. Snowball is a mortal enemy of Animal Farm, therefore anyone caught speaking or communicating with him in any way will be executed.
The farm is now at the service of Napoleon, to be used for his personal agenda, to make money for him and the other pigs as well as to satisfy his desire and need for power.
Squealer might as well stand up and say Comrades, the future is bleak, there is no hope of you ever grazing in the field and enjoying your retirement, you will be used until every drop of life is drained out of you and then if you don't die on the spot, you will be hauled off to the slaughterhouse, that is if you don't get executed for offending the Great Leader!