You also forgot to mention there were “No Trespassing” signs nailed to the trees on the property & that Williams and the other agents he was trespassing on private land with were wearing camouflage head to toe armed with their own firearms, including a suppressed 9mm submachinegun(s).
Because the family dog was leading the Weavers to them, one of them shot the dog. The son, shocked to find a Marshal had just killed his best friend, had a knee jerk reaction and shot one of the camouflaged trespassers. In response, the boy was then shot in the arm and then shot in the back as he ran away.
Let’s not forget about how Lon Horiuchi (the FBI sniper) shot unarmed Vicki (wife/mom) in the face with a .308 round as she was closing the door, holding their baby - and how the surviving family members had to spend the night on the floor by her dead body, with her blood and brains covering the floor as they stayed out of the line of fire. Also, as dawn crept on, the FBI taunted the Weaver family with, “We had pancakes this morning. And what did you have for breakfast?”