Except Koresh didn't burn it, the Feds did, just like the cowardly child killers that some of them are. You could ask the infa-red specialist who was scheduled to testify about it, but he mysteriously ended up dead right before his testimony was scheduled. Try watching the documentary Waco: The Rules of Engagement, or WACO: A New Revelation, and if you have the slightest bit of decency, you'll never make a joke about it again, and you certainly won't think they burned themselves.
Perhaps if you saw the photo's of the little kids curled up in a unnatural convoluted ball after first being gassed, and then incinerated, you might actually have enough class to not make light of such a horrible tragedy and then use it in a failed attempt at humor. That's about as tasteless as the ATF running their flag up the Branch Davidians flagpole right after the carnage, like they had just conquered a foreign country or something while woman and children were still burning.
What's your next joke going to reference? The people jumping off the Twin Towers to escape the unbearable heat, or perhaps you'll reference something more historical like the Jasenovac camp, or perhaps a Jonestown reference or two.