As for the not being rules in warfare ... yes, there are. There are standards - you cannot simply carpet bomb a city
Were I given to sarcasm, I would likely reply "Go tell that to the residents of Dresden!" However, sarcasm never was my scene, and I don't intend to start now.
Instead, Simon, I will ask you which set of international agreements (Geneva, The Hague, etc) prohibit the use of carpet bombing? Terror bombing of civilian targets was a central part of Allied air warfare strategy in World War Two - particularly that carried out by Britain's Royal Air Force. Also, the two atomic bombs were both exploded on civilian targets (the Japanese cities of Hiroshima and Nagasaki). Furthermore, no one was ever tried as a war criminal over any of those acts. Has there been a new set of international agreements drawn up in the 70 years since then that ban carpet the bombing of civilian targets? To the best of my knowledge, there have been none. (Throughout the Cold War, both sides had their strategic nuclear missiles trained on each others cities - i.e. civilian targets. This would tend to indicate that if any such international agreements had in fact been put in place, then nobody ever had any intention of taking a blind bit of notice of these!)
The work of a sniper, of course, is quite the opposite of that carried out by that of weapons of mass killing. Snipers seek out specific individual targets, usually enemy officers - the very antithesis of indiscriminate slaughter.
There are rules of warfare, which cover such things as the treatment of prisoners of war and the status of the wounded. These also prohibit the use of such ammunition as soft tipped / hollow tipped bullets, or the fitting of explosive charges into rounds of less than a certain calibre. Nowhere, though, are there any recognised rules of warfare that start even trying to specify what is "fair", "gentlemanly", "cowardly" or similar!
Bill.
PS: Before anybody starts sending a stream of tomatoes flying in my direction, I am in no way advocating the use of carpet bombing of civilian targets - just merely pointing out that no recognised rules of warfare prohibit its use.