I found the first part of Collapse - the one about ancient societies - to be extremely compelling; the second part is a little too environmentalist for my taste. However, Diamond surely is not an anti-human environmental extremist; he presents rational and valid points and his overall statement is that overexploitation of natural resources will result in societal collapse - which is hardly a false or partisan assertion. Details may vary, the principle is correct.
As I said, I have not read Guns, Germs and Steel but I can anticipate, more or less, one of its themes.
Sometimes, achieveng something first gives one party an advantage that is difficult, if not impossible, to overcome.
For a simple example, if an infantry platoon reaches a hilltop first, it will be difficult to dislodge them, because they will be able to spot and hit attackers (of course, if no powerful fire support is available).
On a more complex case, if one country is the first to develop firearms, it will obtain an advantage over enemies or simple competitors. Can the others start manufacturing firearms themselves? Only if they already have sufficiently advanced metallurgy and chemistry already; a society at the stone-age level cannot build rifles at all - maybe some primitive mortars at best.
So, the stone-age society will survive only if the rifle-armed attackers are magnanimous enough to let the others survive.
The technological advantage is morally neutral; whether to let the less advanced ones survive or not is the ethical choice.