If you look at what happened, its clear that Alderaan was a thug, and not in that system to buy skittles and an Arizona Iced Tea. It was looking at other planets, very suspiciously, and it had marijuana smoke in its atmosphere. The Death Star was just doing its duty, even if the police said to wait, in tracking down Alderaan. When attacked the Death Star did what it had to, as Alderaan was bashing the head of the Death Star on the ground. It was justified.