Mind precedes all mental states.
Mind is their chief; they are all mind-wrought. 
If with an impure mind a person speaks or acts, 
suffering follows him like the wheel that follows the foot of the ox.

Mind precedes all mental states. 
Mind is their chief; they are all mind-wrought. 
If with a pure mind a person speaks or acts, 
happiness follows him like his never-departing shadow. 
Dhammapada verse 1 and 2

It is intention [or volition; instead of just an action], monks, that I call kamma [karma],
for having willed, one performs an action through body, speech and mind.

I am the owner of my karma .
I inherit my karma.
I am born of my karma.
I am related to my karma.
I live supported by my karma.
Whatever karma I create, whether good or evil, that I shall inherit.
Anguttara Nikaya v.57 - Upajjhatthana Sutta

Verily, an evil deed committed does not immediately bear fruit, just as milk curdles not at once;
smouldering, it follows the fool like fire covered with ashes.
To his ruin, indeed, the fool gains knowledge and fame;
they destroy his bright lot and cleave his head.
Dhammapada v. 72 and 73

Do not overlook negative actions merely because they are small;
however small a spark may be, it can burn down a haystack as big as a mountain.

If you try to subdue your selfish motives—anger and so forth—and develop more kindness and compassion for others, ultimately you yourself will benefit more than you would otherwise. So sometimes I say that the wise selfish person should practice this way. Foolish selfish people are always thinking of themselves, and the result is negative. Wise selfish people think of others, help others as much as they can, and the result is that they too receive benefit.

The universe that we inhabit and our shared perception of it are the results of a common karma. Likewise, the places that we will experience in future rebirths will be the outcome of the karma that we share with the other beings living there. The actions of each of us, human or nonhuman, have contributed to the world in which we live. We all have a common responsibility for our world and are connected with everything in it.

We have the ability and the responsibility to choose to direct our actions on a virtuous path.
When we weigh a particular act, to determine whether it is moral or spiritual, our criterion should be the quality of our motivation. When someone deliberately makes a resolution not to steal, if he or she is simply motivated by the fear of getting caught and being punished by the law, it is doubtful whether engaging in that resolution is a moral act, since moral considerations have not dictated his or her choice.
In another instance, the resolution not to steal may be motivated by fear of public opinion: "What would my friends and neighbors think? All would scorn me. I would become an outcast." Though the act of making a resolution may be positive, whether it is a moral act is again doubtful.
Now, the same resolution may be taken with the thought "If I steal, I am acting against the divine law of God." Someone else may think, "Stealing is nonvirtuous; it causes others to suffer." When such considerations motivate one, the resolution is moral or ethical; it is also spiritual. In the practice of Buddha's doctrine, if your underlying consideration in avoiding a nonvirtuous act is that it would thwart your attainment of a state transcending sorrow, such restraint is a moral act.
An Open Heart: Practicing Compassion in Everyday Life

Sometimes we face certain situations where, although we have done something good for others, we may not be able to reap the consequences within this lifetime. When we are talking about the law of causality, we are not limiting its operation to the confines of this life alone, but rather are taking into account both this lifetime and the future. Occasionally people who do not have a proper knowledge of karmic law say that such and such a person is very kind and religious and so forth, but he always has problems, whereas so and so is very deceptive and negative, frequently indulging in negative actions, but always seems very successful. Such people may think that there is no karmic law at all. There are others who go to the other extreme and become superstitious, thinking that when someone experiences illness, it is all due to harmful spirits.... It is also possible for very negative people to experience their positive karma ripening immaturely due to the very strong force of negative actions, and thus to exhaust the potentials of their virtuous actions. They experience a relative success in this life, while others who are very serious practitioners, as a result of the force of their practices, bring upon this lifetime the consequences of karmic actions which might have otherwise thrown them into rebirth in lower realms of existence in the future. As a result, they experience more problems and illnesses in this life.
Just resolving not to indulge in a negative action is not enough. It should be accompanied by the understanding that it is for your own benefit and sake that you must live with awareness of the law of karma: if you have accumulated the causes, you will have to face the consequences; if you desire a particular effect, you can work to produce its causes; and if you do not desire a certain consequence, you can avoid engaging in actions that will bring it about. You should reflect upon the law of causality as follows: that there is a definite relation between causes and effects; that actions not committed will never produce an effect; and that once committed, actions will never lose their potentiality simply through the passage of time. So, if you wish to enjoy desirable fruits, you should work for the accumulation of the appropriate causes, and if you want to avoid undesirable consequences, you should not accumulate their causes.... [Karma] is a natural law like any other natural law.
Path to Bliss: A Practical Guide to Stages of Meditation

Some people misunderstand the concept of karma. They take the Buddha's doctrine of the law of causality to mean that all is predetermined, that there is nothing the individual can do.
This is a total misunderstanding.
The very term karma or action is a term of active force, which indicates that future events are within your hands. Since action is a phenomenon that is committed by a person, a living being, it is within your own hands whether or not you engage in actions.
Path to Bliss: A Practical Guide to Stages of Meditation

Sometimes, you may encounter situations that require strong countermeasures. I believe, however, that you can take a strong stand and even take strong countermeasures out of a feeling of compassion, or a sense of concern for the other, rather than out of anger. One of the reasons why there is a need to adopt a very strong countermeasure against someone is that if you let it pass--whatever the harm or the crime that is being perpetrated against you--then there is a danger of that person's habituating in a very negative way, which, in reality, will cause that individual's own downfall and is very destructive in the long run for the individual himself or herself. Therefore a strong countermeasure is necessary, but with this thought in mind, you can do it out of compassion and concern for that individual.
The Art of Happiness: A Handbook