Question

why is it necessary to kill pupa by boiling cocoons in water

Answer 1

the caterpillar spins a cocoon and spends its pupal stage inside. Cocoons need to be unravelled to get silk. After growing, the caterpillar which must have turned into a butterfly by undergoing metamorphosis must eat its way out the cocoon. So there is very little possibility of getting silk from such a cocoon. To get silk, the cocoons are put in boiling water during the process of which pupae die.
                If in short we wnt to say we can say that:
To make silk 

Answer 2

 it is necessary to kill pupa by boiling cocoons in water  so that it does not break