Rain, you live in a world full of insane people by that definition
Seriously, to correct one point I need to redact:
The Geneva Conventions have very different flavours over the years. If you just take the basic four, or the first three, they have a very different feel to when you've added on Protocols I and II. The US is not signatory to protocols I and II. So, if you signed the first 3 or 4, but not the last two amendments, you may well have a very different view on how irregulars/terrorists/illegal combatants should be handled in OotW.
I'll agree with the sentiment that the Imperium must have a number of laws. At the same time, we have canonical evidence that they are willing to be fairly brutal against enemies and even revolutionaries. So, given this is so, have they
A) Writen some rules and then ignored them
or
B) are the rules written to allow this kind of thing including orbital bombardment and the like?
Furthermore what are Vilani sentiments like? What are other human minor races and non-human minor races to contribute? How much do Solomani historical values play into things? This is the kind of thing one would need to think about.
I'm fairly certain the original Geneva Conventions came about to ensure the protection of captured prisoners who were uniformed combatants. They were agreed upon by powers of equal nature (all human nations with roughly the same concerns) such that reciprocity was a key part of the underlying logic and raison d'etre.
How does this apply in a TU? We've got other large national entities like the SWC, the ZC, the SC, etc. and with them, you'd expect some form of such conventions to be established. but you've also got a lot of less predictable enemies - pirates and privateers, local insurgents, the Vargr, the K'kree, etc, for whom reciprocity might not happen and who may well not respect or honour any treaties signed anyway. So, how do you apply the LoW then? How do you write the LoW to handle these situations? Something tells me in 58 centuries, there would be some good solutions that wouldn't leave the Imperium fighting with one hand tied behind its back.