For the most part karma and LoA are misunderstood concepts. While LoA is not at its root a concept, the perception of it generally is. My sense is that karma is more concept than not.
1. If something "bad" happens to me then it must mean that I did something "bad" in my past?
No. If something 'bad'
happens to you it's likely because you hold a perception that some things 'should be'
and others 'should not'
. The 'bad' stuff is simply the stuff we don't want and resist. That resistance to what is is the quality that makes circumstances feel painful and thus 'bad'. We create 'bad' by our applied perception. It is not an inherent quality.
2. It seems to me that Karma is a personal viewpoint, a way of justifying the world. To make sure everything is fair and even in the long run.
As a concept, that is much of what the perception of Karma has devolved into. And one can create a world view (many have) based on such a concept and perception. As your friends have demonstrated, it gives one the feeling and satisfaction of Cosmic payback. In a universe based in Love and expansion however, it's just not how things work. Consider, who exactly would payback-karma be for? The ego perspective that persists only while humanly focused? A perspective that likely evaporates through the awakening to a greater reality of being in the process we call death?
As souls, our true nature, we explore possibilities of being. The human experience is just one such possibility. Perspectives don't learn. Consciousness does. Perspectives are a result of experience and the assumptions made from that experience. Karma, if anything, is the resulting experience of conditions based on our adopted assumptions/beliefs.
3. For every "good" you do won't someone consider it "bad?" For example, if I mowed the elderly lady's lawn for free, the person who makes a living off mowing lawns will consider it bad for business.
It's quite possible. These considerations of good and bad are based on one's egoic beliefs and perspectives toward life. Our experience will always be reflected through the lens our beliefs. When one is clear on this (which then changes core beliefs), one can begin to make different/better choices in how to perceive events, thus changing one's experience in life or one's karma.
4. Who judges what is "good" and what is "bad?"
Each of us does our own judging through our own egoic perspective. A perspective of judgment is born of an extremely limited point of view.
From the Soul perspective, no judgment is made. Only Love and appreciation for its own extension and child of being. We knew, in our Soul perspective that life in this human world would be challenging. We knew that we would be entrained with misconceptions, and that those misconceptions could create painful experiences through misdirected choices. But we also knew that there was much to be gained in the expansion of consciousness through the challenge of creating a more joyful human experience - and that however life unfolded, facing the challenge alone would bring great benefit.
There was never going to be any fault to be cast from this Origin of human-life Soul, for however the human experience unfolded. Human being is after all, a fragment of the Soul's own Self that is living the human experience. Why would it offer anything but Love?