From my understanding of the event, Sammy shot at them. But they were suspicious trespassers on his property, who had just slaughtered his dog. He had every single right to fire upon them, both legally and logically. Him being shot in the back was an attempted retreat as well. The Marshall's literally went there to "test" the dogs, and when one reacted like a fucking dog did they were shocked that the two guys followed the dog's barking.

In fact, they knew from the start they had fucked up bad. That's why they went into hard denial of even pretending anyone shot at the boy. Including the FBI "evidence" collection missing numerous bullets that the local sheriff office was able to just wander around and find later.

Like, every single aspect of the Ruby Ridge story can only be justified if you think the government is above the law and can do whatever they want to you, and you are supposed to grovel for their mercy or else they will rain hell upon you.