"I mean, yes, they had illegal weapons"
You realize of course that the ATF is (was) a tax collection agency under the treasury dept. They are now under Homeland Security.
Any "illegal" weapons they may have had were only illegal because of lack of payment of a $200 tax.
David Koresh was personally known by the local Sherrif, and could easily have been arrested on one of his regular trips to town.
80 people were killed over non-payment of tax and there has never been any proof that any of the weapons were illegal.
Posse Comititus(sp) was violated as well.