I don't think the OP was asking or inferring that anyone intentionally brought down WTC 7 after the fact.
The OP seems to ask how - given relatively small fires - the building could have collapsed so symmetrically. That is also what experienced structural engineers have asked, and probably many are still trying to understand what happened.
Clearly the buildings in question experienced situations beyond design. And in fact, one will find that the 'normal' design process is often deficient in addressing beyond design situations, which are considered quite rare.
I should have pointed out that the buckling of the building coincided with the longest members, and that is where one would expect it to fail - i.e. no surprise there.
The other point - structural analysis, and particularly predictive analysis is very complex. There are very few who do it very well.