Then, three military helicopters flown by the Texas National Guard arrived at the scene. One was a UH-60 Blackhawk, a large helicopter that is among the military's most advanced. And, yes, its standard paint job is a dark shade close to black. Just how ATF personnel ended up riding in military helicopters is puzzling in itself. These accusations enabled it to invoke a regulation allowing military participation in anti-drug raids.
Official ego seems the likely reason.
Imagine what the Davidians, known to share the far right's peculiar dread of government helicopters, must have thought when they saw a Blackhawk approaching. The silhouettes of military helicopters are distinctive and easily recognizable, particularly to weapons buffs such as the Davidians. The Blackhawk used in the Waco raid was not armed, but while it is possible to tell that a helicopter is military from its silhouette, a glance will not necessarily tell a ground observer whether that helicopter is armed. The arrival of the military helicopters seems to be what transformed Waco from a case of overzealousness into a tragedy. Until that point in the operation, neither the ATF agents nor the Davidians had pulled the trigger.