The WTC collapse issue is possibly the most simple one there is. The single, and only, reason Conspiracy Theorists offer their denial of the collapse is as follows:
They are ignorant of, or in denial of, the simple fact that tall structures are inherently unstable.
The perception is of a structure that is strong and robust, quite happy to remain where it is, that requires the injection of enormous amounts of energy in order to force it to move out of its natural state and collapse.
They have it all backwards.
A building wants to collapse. On every building, at all times, there is a constant and irresistible force urging it to collapse. The fundamental effort of engineering is to fight against that urge to collapse. Disrupt the structure, even just a surprisingly small amount, and that urge takes control, and the building comes down - returning to its natural state - collapsed.