What happens when someone has dark intentions in their thoughts, but does not act on them? Can it actually bring harm to others or to the self?

The holding of a dark intention for harm to another solely within one’s own thoughts is not a benign act of no consequence. Because thoughts are things, they are an energy and birthed into existence and they will extend beyond the individual because thoughts are designed to be funneled into the collective unconscious. But at the same time you create an intention and focus it to a particular topic and have a certain emotional investment in its meaning and assign that to a target, for example, another human, you have all the ingredients of a psychic attack and precisely because there is animosity with dark intentions, that will add extra energy and force and will seek its target. Whether the person holding those intentions knows this or not, the other party will get the message intuitively and will feel it within their being as a disquiet, as a type of inner stress. It will be ill-defined in most cases; very few people have the high-level psychic ability to read others’ thoughts and know when a thought from someone is impinging on them and can deduce the originator and the message both, but most people will be sensitive to not being liked and will sense that energy emanating from someone even at a distance—it will enter their subconscious and influence their emotional state and have a consequence on the overall mood, level of happiness, and even potentially the triggering of a nightmare. Such psychic attacks if done over a period of time, particularly, can even cause illness within a person. So, far from being a harmless exercise, it is, in effect, playing with a loaded weapon, albeit one seemingly much less impressive than a firearm, but a weapon nonetheless. At the same time it can potentially harm another, it will always harm the self because time spent with dark thoughts creates a karmic potential to actually send that harm to others, and that will be assigned to the originator as a burden of responsibility for anything that might happen as a consequence. It will also diminish the self because in generating the dark intention, you will be moving out of alignment with the divine and causing a diminishment of your soul potential, at least in the moment, and to have dark notions assigned within the akashic records because that is how you spent your day or period of time wallowing in the mud, so to speak. There will be an energetic signature of misalignment and negativity needing to be rebalanced at some future time, and that will come back to haunt you at some point demanding a repayment. So it is particularly the case when this becomes a habit, and for some it is a way of life, and in the doing the person is digging a hole for themselves and the Law of Karma will make that clear at some point with their life taking a turn for the worse and they begin to experience the very negativity they were launching against others, because it will circle back to themselves as a corresponding penalty. The person will be facing not only the level of negativity of the original intention but now compounded by the consequences it generated in finding its way to the target being thought about, and perhaps injuring them, and causing harm even to others as a result of perturbing the target in some way. All of the negative outcome will be reassigned back to you courtesy of the Law of Cause and Effect and will become your burden to deal with. This shows the wisdom of cultivating a higher vibration; it is akin to the conventional wisdom, "If you have nothing nice to say, then say nothing at all;" the same wisdom applies to one’s thoughts and intentions.