Blake challenges powerful people in their ‘palaces' which are marked by the blood of good people. Why is the speaker angry with the soldiers being killed?

Because he believes that soldiers are killed in a fight that they did not provoke, but were forced to attend.

Explanation:

For Blake, wars were caused by the nobility and the powerful in favor of their own goals, but they did not sacrifice themselves in wars, but summoned the good men, workers and providers of their families, to fight in a war that was not theirs and still being killed and wounded in combat, causing great suffering for them and their families. This left Blake bravado and disappointed in the society in which he lived.