The FBI screwed up, but both the US Marshals and the FBI were handed these bag of worms by the ATF. The firearms charge that Weaver was out on bail was that he shortened a shotgun barrel more than legally allowed for an ATF confidential informant, so argueably a case of entrapment. They also informed the Marshals that Weaver was a suspect in an armed bank robbery and was armed and dangerous, which wasn't true. Things went downhill from there.

The same at Waco, the Texas Rangers requested that they be allowed to wait and pick him up the next time Koresh came to town, instead the ATF chose to make a highly public assault on the compound.

Koresh was a wacko, but there never was much evidence that he was a danger to society. The stuff about the kids didn't come out until a lot of people were trying to cover their butts, so I take it with a grain of salt.

Yes, Weaver was a survivalist and a white supremist, but through all of this stuff there never was allogation that he was even a member of groups like the Aryan Nation. It was never clear though any of this why he even come up on the ATF radar.