Is the force that ender used on Stinson justified?

No, beating someone to death can never be justified, particularly in the situation between Ender and Stinson. None-the-less, the people in charge orchestrated the event "hoping" Ender would have the capability to do exactly what he did. So, you would find your answer in your own opinion...... should Ender (remember he's only six) be capable of complete self-control..... or was he pushed beyond his limits?