@PervySageChuck
Historically speaking, it's the WINNERS of a war who determine who will be charged with war crimes. As shown by the results of WWII when the Germans lost and were charged with numerous war crimes, but the British and the Americans were NOT charged with the crimes of firebombing the cities in Germany and killing untold numbers of civilians in the process.
... or in the case of America, nuking Japanese cities full of civilians.
I think the real reason why she's trying to abide by the laws of war is different.
If the Empire is too obvious with breaking international laws, they'll give every other country plenty of reasons to go to war against them. They're already struggling with keeping Britain off their case, and America's right around the corner.
If Tanya were the reason why other countries ally against the Empire, she might face trouble within her own country. Worse if they lose, because she'll be judged for those crimes.
Even the current situation puts her in a very sticky situation, because once the Empire loses, the remaining countries could still charge her for twisting the definitions of international laws. She probably knows that, which is why she's uncomfortable with putting her plan into action. At this point, all she can hope for is the Empire winning the war, because she'll have a hard time justifying herself if they don't.