There were a lot of design flaws and skirting of building codes when the World Trade Center was built.

The steel columns were supposed to be covered with asbestos, but that was only done part way up on the South Tower.

For a good description of the flaws, and why the fire (in an enclosed space and fed in part by oxygen coming up the shafts) did ultimately cause the failure of the materials, see
http://www.civil.usyd.edu.au/wtc.shtml

While the fire would not have been hot enough to melt any of the steel, the strength of the steel drops markedly with prolonged exposure to fire, while the elastic modulus of the steel reduces (stiffness drops), increasing deflections.

Modern structures are designed to resist fire for a specific length of time. Safety features such as fire retarding materials and sprinkler systems help to contain fires, help extinguish flames, or prevent steel from being exposed to excessively high temperatures. This gives occupants time to escape and allow fire fighters to extinguish blazes, before the building is catastrophically damaged.

It is possible that the blaze, started by jet fuel and then engulfing the contents of the offices, in a highly confined area, generated fire conditions significantly more severe than those anticipated in a typical office fire. These conditions may have overcome the building's fire defences considerably faster than expected. It is likely that the water pipes that supplied the fire sprinklers were severed by the plane impact, and much of the fire protective material, designed to stop the steel from being heated and losing strength, was blown off by the blast at impact.

Eventually, the loss of strength and stiffness of the materials resulting from the fire, combined with the initial impact damage, would have caused a failure of the truss system supporting a floor, or the remaining perimeter columns, or even the internal core, or some combination. Failure of the flooring system would have subsequently allowed the perimeter columns to buckle outwards. Regardless of which of these possibilities actually occurred, it would have resulted in the complete collapse of at least one complete storey at the level of impact.

Once one storey collapsed all floors above would have begun to fall.

It's difficult for me to believe in a conspiracy, given so many competing agencies and professionals studying the collapse of the towers from all kinds of different angles--materials science, fluid dynamics, physics, etc.  There really was a pretty good consensus.

It's a little like the hypothesis about global climate change.  Slowly a body of agreed-upon facts from many different sources has accumulated, peer-reviewed papers have been published, and dissenters have made their cases.  Overall at this point the conclusion of the scientific community is that the rise in global temperature is real.  People are still arguing over the details, though.

Like any scientific hypothesis, the explanation of the collapse will no doubt evolve as more information, more angles on it, and perhaps better technology arise.
Further questioning can't hurt.  We're not talking theology here.