Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
Harry would likely be defiant at first. He has a strong sense of justice and would not easily submit to Voldemort's punishments. For example, if Voldemort uses the Cruciatus Curse, Harry would probably try to resist the pain and not show weakness.
He'd probably be furious. Voldemort has a huge ego and most fanfiction would likely not portray him in the way he sees himself. He might view it as a form of disrespect to his 'true' nature and power.
Voldemort would be calculating. He might be using the Armistice as a way to regain his strength or plot a new strategy. His Death Eaters would be divided. Some might be happy to have a break from the fighting, while others would be eager to continue the war. Dumbledore's followers would be watching closely too. They would be the ones to keep the balance and make sure that the Armistice doesn't turn into a way for Voldemort to take over completely. The Ministry of Magic would be in a difficult position, trying to enforce the Armistice while also dealing with the different factions within the wizarding world.
Ron would be extremely angry at first. He might feel betrayed by Harry. But as he realizes that something must be seriously wrong with Harry, he would probably join Hermione in trying to save Harry from Voldemort's influence. They would look for a way to break whatever spell or false belief has hold of Harry.
The Death Eaters would initially be overjoyed. They would think that with Harry on their side, they are invincible. But some of them might also be suspicious of Harry's motives, fearing that he is a double - agent. As for the general wizarding population, they would be terrified. Harry was seen as their hope, and now that he has joined Voldemort, they would feel completely hopeless and defenseless.
The wizarding world would be in chaos. The Order of the Phoenix would be extremely worried and likely try to rescue Harry. The Death Eaters, on the other hand, might see it as a sign of Voldemort's power and dominance. Hogwarts would also be affected, with teachers and students fearing for the future of the wizarding world.
The other Hogwarts students would be a mix of scared and curious. Some might avoid Harry altogether, fearing that they could be in danger. Others, who are more brave or naive, might try to approach him to see if they can help. The teachers would be in a difficult position. They would need to protect the other students but also try to find a solution for Harry. For instance, Professor McGonagall might be torn between her loyalty to Harry and her duty to keep the school safe.
Hermione would be horrified. She would immediately start trying to figure out a way to rescue Harry. She might start researching in the library for some ancient spells or forgotten magic that could help. Ron would be angry at Dumbledore and would probably want to rush off to save Harry without much of a plan at first.
Well, the pure - blood families who were on the fence about Harry might finally accept him wholeheartedly. They would see that he was the one who could truly rid the wizarding world of Voldemort. However, there would also be those who would be critical. Some of the more traditional wizards and witches might think that using a shooting method was too violent and un - wizard - like. They might worry that this sets a bad precedent for future conflicts in the wizarding world. But overall, the general public would be relieved that Voldemort was finally gone and would likely celebrate Harry as a savior.