I am Second Senior
... if they want to kill all humans.... ... and can sneak generals into a city... ... why not randomly attack any/all the time... other than MC, seems like the demon generals can escape from anyone else, so no real danger to high level forces, but taking out food, shelter, industrial areas, or just civilians to cause panic with no real danger to high level forces seems... ideal and yeah, humans could team up, but targeted missions <even guerrilla tactics> and a chance to take out (lone)human masters seems worth it