David Koresh (or more accurately Vernon Wayne Howell) was a nut job. He claimed to have the gift of prophesy, said he was destined to father a child with the 77-year-old leader of the Branch Dividian sect, and that their child would be "the chosen one."
Yeah, I think "nut-job" is being kind.
The ATF had a legal warrant based on several complaints. One area dealt with child abuse/molesting (he is known to have had several underage "brides"), and the other dealt with neighbor reports of the sounds of automatic firearms and Koresh illegally converting AR-15's into M4/M16 FA rifles.
Based on the warrant, he needed to be arrested. After the arrest, he could post bail, defend himself against the charges, provide evidence to clear his name ... whatever else.