Once the agents were killed, there was no turning back. This should NOT have ended with an inferno.
Unfortunately, you are right.
Howell and his followers thought they were under attack. At one point, he even called 911.
A friend of mine followed the whole thing very closely. He believed that the whole thing went south when BATF agents executed dogs that were either attacking them or believed to be dangerous.
Those initial gunshots apparently set off the fire-fight. It isn't clear who over-reacted to the shots first, but I think my friend is right about what triggered it.