and my question is: I can't do harm upon innocents? who is going to stop me from mass murdering a bunch of people?
How do you decide whether a person is innocent? We might see cases where a baby may die right after birth. From our point of view that baby should be innocent, because she had no chance to do something wrong. But in reality, our view is very limited, because we only see the physical body of a baby, but not her soul, which also carries the Karma of past lives. So, if we have insufficient information about a person, how can we judge people and what is happening to them?
In contrast: enlightened beings, like Bodhisattvas, can see more than the three physical dimensions. They can look at a person and see the MILLIONS of years that the person have spent on this planet and also the Karma that the soul has accrued on the way. These kind of beings, and even way higher kind of beings/intelligences, may be involved in sorting out Karmic debts and how they are paid.
God won't lift a finger and we know it well, as these mass genocides happened many times and they will keep happening.
The mass genocides have happened in the past and this is why this humanity has been judged for its' perversity. The judgment has been decided upon and this humanity will be destroyed, otherwise humanity won't stop repeating their evil deeds. BUT God is giving this humanity a last chance to rectify their mistakes and additionally, God grants this humanity free will, the right to choose. If you see perverse events happening in the world, you simply see the choice of the people. Everyone has a choice in every moment of their life and we see how this freedom to choose is being misused.
can harm be done upon innocents? for me it seems like. I don't see what would stop me or anybody else.
A normal, healthy person would be stopped by their conscience. Because a normal person has a connection their inner divinity, their Innermost Being. And through conscience a person is able to "do the right thing", or abstain from wrong deeds.
The sad situation is, that right now there are a vast number of people, persons who look normal on the outside, who have no connection to their Innermost Being anymore. They have a humanoid body, but are basically vessels for evilness. This is why they do not feel any conscience, remorse, compassion. Such a person is capable of murders, etc. and there is no Inner Being to stop them. In such a case, the Lords of Karma would intervene, to make sure that no one is harmed, who is truly innocent or whose judgement day has not yet come.
There are many more aspects to Karma and it's because Karma is a very, VERY complicated issue. If we are not a Lord of Karma, let us realize that we do not fully comprehend the full picture and that our discussion is merely intellectual/theoretical (or probably not correct).
I would rather encourage you to reflect on your emotions, which are visible within your questions. Negative emotions are responsible for the atrocities that we see right now. And it might be that we also have such Karma waiting for us already. As soon as this Karma starts moving, we will have no time to clearly think it out, we will have to choose quickly. So, let us study our own negative emotions, dissolve them and create virtues. This will give us the right tools to overcome such Karma. Feel free to study this:
https://glorian.org/books/the-revolution-of-the-dialectic/anger
https://glorian.org/learn/courses-and-lectures/lectures-by-samael-aun-weor/alcyone-and-negative-emotions