Karma, imo, is the consequence of decisions we make. It is the consequence of other's decisions, as well.
I believe we choose people, to be in our lives, who are as emotionally healthy as we are.
When we allow people into our lives - hobby or otherwise, and draw healthy emotional and physical boundaries - we demonstrate our own emotional health.
When we allow people into our lives - hobby or otherwise, and the relationship becomes about rescuing, controlling, playing mind games, continual dishonesty, jealousy and so on...and it ends badly - we
invite that drama into our lives.
If you get screwed over by someone and then, they get screwed over by circumstances or someone...that 'payback' isn't karma - it is a natural consequence of treating others like crap or living a life w/ no accountability.
It would be easy to excuse our screw-ups and negative consequences to happenstance or being a victim, while at the same time, delighting in the fact that 'karma' finally caught up w/ someone and they got what was coming to them.