First and foremost it's important to know karma is not so simply described as do good things and good things will happen to you or vice versa bad things. Though in that standard description there are a few subtle elements of truth. First off karma is entirely action based, it doesn't matter why you do what you do, merely what you've done in itself. So murder even in self defense is still murder and thus so are the consequences. Even rewards are consequences, keep in mind that a consequence is not inherently bad it's merely the result. Next is that deeds do not erase or override other deeds there isn't some overall balance in play, each deed runs separately. Furthermore if you could so easily separate deeds by good and bad, then for example bad deeds do have an expiration, it's not as though it will follow you to the end of days.
Another widely held misconception is that somehow the world is conspiring or the universe is adjusting based on your deeds to deal out the consequences of your actions. The truth is merely that you are the sole proprietor of your rewards or punishments if you would dare to separate them as such.
Though some Hindu beliefs do contradict this, it's important to remember that karma is not native nor exclusive to Hinduism. In particular they do believe in a just and fair deity that is responsible for your karma, and while specific actions have specific consequences they also adjust a secondary overall balance of karma which can determine a baseline for your life, in particular when being re-incarnated.
I suppose most of the confusion on the subject does come from the many forms or ideas of the concept that are present in the world. For example the Buddhists do believe your actions are very separated into both good and evil but the consequences are exclusive to that which draws you closer to nirvana or keeps you stuck here on earth in the cycle of re-birth. Then there's Jainism which is where people get the idea the world is changing based on your actions. This is because Jainism would have you believe karma is a field that pervades the universe and the particles are attracted to soul fields and the stronger your field the more particles are attracted to you. This overall balance of particles and the strength of your field determine the world you live in. Furthermore it also states that karma is not just action based but also thought based which is a major difference to consider as it's the only form to do so.
So with all these different versions of karma to consider, what's really universal, what can you say is true of karma across the board. The answer is that you reap what you sow, in this life, and possibly more.
Really though you shouldn't have to rely on some sense of spirituality to do the right thing, or as a fearful reason to not do what you want or would enjoy. In the end it's up to you to determine what's acceptable to you. Come what may I will do as I please and I do not fear the consequences because I am safe in my knowledge that I have done what I thought was right regardless.