You are leaving out a very important piece of detail. The reason he was sentenced to death is that he was convicted of murdering someone during the protest. You can argue that he is not the murderer and it was pinned on him. You might be even right. But it is undeniable that someone was murdered in the protest, and that his sentence was because he was convicted of his murder. Had there not been a dead body on the ground, he would not have received a death sentence.