There are tales of Rakshasas (Demi gods) in Mahabharata (Kansa, etc), Ramayana, etc. who committed mass murders.
Did they not acquire bad Karmas for what they did?
All beings acquire karma except the ones who have realized their true nature. For them alone, there is no karma.
Complete Works of Swami Vivekananda / Volume 3 / Lectures from Colombo to Almora / Vedantism:
Persons who do good works here with the thought of reward, when they die, are born again as gods in one of these heavens, as Indra and others. These gods are the names of certain states. They also had been men, and by good work they have become gods; and those different names that you read of, such as Indra and so on, are not the names of the same person. There will be thousands of Indras. Nahusha was a great king, and when he died, he became Indra. It is a position; one soul becomes high and takes the Indra position and remains in it only a certain time; he then dies and is born again as man. But the human body is the highest of all. Some of the gods may try to go higher and give up all ideas of enjoyment in heavens; but, as in this world, wealth and position and enjoyment delude the vast majority, so do most of the gods become deluded also, and after working out their good Karma, they fall down and become human beings again. This earth, therefore, is the Karma Bhumi; it is this earth from which we attain to liberation. So even these heavens are not worth attaining to.