The most likely reason why Tommy shouted, “It’s the monster! It’s the monster!” is because fear was taking over his mind and every little movement or action seemed suspicious or threatening.
Tommy is a young boy, and his imagination is very active. In a situation like this, it's easy for him to misinterpret what he's seeing and hearing. The shadows of the trees, the rustling of the leaves, and the creaking of the wind can all sound like strange and threatening noises.
And if Tommy is already feeling scared, it's easy for him to see these noises as signs that something dangerous is lurking in the darkness.
Why does Tommy shouts, “It’s the monster! It’s the monster!” ?
Because he saw a monster slowly walking toward them
He wanted Charlie to kill Pete Van Horn
Because fear was taking over his mind and every little movement or action seemed suspicious or threatening
He was the monster and wanted to distract the neighbors from Maple Street