Belief in magic is a choice. One can choose to believe in magic or one can believe magic is just fantasy. Just because some people do not believe in magic does not mean that magic doesn't exist. Any intelligent person knows that "for every action there is and equal and opposite reaction". This is a part of the science of physics. Karma is the spiritual version and exists whether you choose to believe in it or not.
If you choose to throw stones and do not believe in karma that is fine. You can explain the stones thrown at you as a reason to throw more stones then. This will keep you busy while the rest of us actually do something productive with our lives like taking the stones you threw and building a stone structure that will be useful for years to come. ;) If you choose not to believe in "what you sow so shall you reap" you can see the structure from the inside. All that stone trowing caused you to not have what you needed and you got caught stealing what you "needed". Then you can have the belief that you can throw more stones without having to use anything but your mind.
If you choose to cast magic that is considered by some as black, evil, etc. you need to be much more aware of karma then those who do not cast such magic. I say this from experience in casting magic that some describe as dark. I personally believe that one can cast what they want to, however that does not mean there are not consequences to the actions. When you cast magic that is negative there are steps that need to be taken. First and foremost ask yourself is the action worth the price? Is there a way I can avoid negative repercussions? What is the best course of action? Don't forget to place enough protection magic on yourself and all you hold dear before you cast magic that could been described as black.
I suggest actually taking the time to study magic and understand it so that you will be familiar with all aspects. To fully understand how to break a curse on must know how to throw one.
Because for every know it all and doesn't care caster there are many others who are actually trying to learn magic. Those are the ones I personal speak to. They are the silent ones that read, take note, and are effected by my words. ;)
When people refer to Newtons third law of motion (the action-reaction law) I don't think that many of them are aware that it is a mechanical law i.e. it refers to forces, not energy. A force being any impulse that changes an objects speed, shape or direction. Sorry for the impromptu science lecture, but seeing as most people refer to magic as the manipulation of energies, the laws of motion aren't really relavent in this case.
Also, if good and bad are subjective, does that mean that if I think an action is good and do it I will recieve good karma? Even if that action were something considered evil by others, such as the murder of an innocent. Just a thought.
You know people who do things like murder innocents usually do in fact think what they are doing is right.
Take any religious war or actually any case involving a seriel killer.
Humans justify their actions.
Not everyones brain works the same so not everyone thinks the same.
And if the don't believe or care about karma or whatever it is you want to call it then if and when anything does happen most likely they aren't going to care either way.
Everyone is allowed to think what they want and if their ideas don't fit into your ideas of "right and wrong", oh well. Leave them alone and concern yourself with your own life.
Lets just call it the way it is.... Everybody has there own views on what they believe... So why tell someone they can't believe in it...It's like the old saying.. Opinions are like belly buttons there all different. Why would anybody want to rain on someone else opinion is beyond me.