I don't believe in karma but I do believe in sociology :-)
My father told my about this study in birds on an island.
The birds would get some burrowing insects on them and if left it would cause death.
The scientists studying the birds identified initially 2 groups. They named them suckers and cheaters.
The suckers would use their beaks to take the insects out of the feathers of any bird who asked. Then they expected the other bird to reciprocate. Obviously when two suckers got together it was a reciprocal arrangement.
Cheaters let a sucker de-insect them then buggered off.
The scientists realised they had missed something as when you model the maths this behaviour would ultimately result in extinction.
So they studied the bird population again and by tagging the birds found a new group.
They called them grudgers. This group would initially assume any de-bugging was a mutual arrangement and any first encounter with a new bird was likely to be positive. However when they came across a cheater they didn't attack they simply refused to de-bug that bird again
They also found that most suckers morphed into grudgers as a result of repeated exposure to cheaters.
Equally some cheaters became suckers (but not grudgers) in an attempt to re-integrate into the colony after repeated attempts to de-bug were refused. Most cheaters didn't adapt their behaviour and died.
It was the group of grudgers who effectively ensured the survival of the colony of birds.
It's a long way of saying that you reap what you sow. If you keep fucking people over eventually you'll look around and realise no one wants to help you or even cares.
Equally if you have been fucked over you have a choice to still trust people, but never again let people walk over you.