And that's just where your mistakes start. No, they are genuine beliefs of these people that their acts are necessary and thus not evil. Very few people are intentionally evil, and most of those are so reluctantly.
If you examine the most evil historical people you invariably find a crapton of self-justification and rationalisation why they had to do what they did and how it was for the good of all. The most evil people in existence are those fully convinced that what they're doing is right to the point that they no longer check.
Most is better than all. You're constantly engaging in the false dilemma that the only options here are "save everybody" or "kill everybody", rather than the more reasonable "save as many as you can".
Actually, if it only got there a few days before, it'd likely not even have seen a mill yet, especially during harvest time. Grain is typically stored for use later, when fresh produce isn't as available, not consumed immediately, which also necessitates that you use older stores before starting on the fresh deliveries.
For that matter, it's rarely eaten whole. Inventoring grain for a whole city would take a while, as would milling it and turning it into baked and other goods.
It's actually rather unrealistic that anybody partook in it any earlier than the morning Arthas arrived if it only arrived two days earlier.