As to Buddhism killing is wrong and planning such things is also wrong. So what is the difference between the Karma of a person who killed someone and a person who only planned but never killed.
Je Tsongkhapa differentiates between (1) karma that is done (2) and karma that is accumulated. Between the two, there are four possibilities.
1. Done, but not accumulated. Tsongkhapa states:
Killing that is karma which you have done but not accumulated is seen in the following cases: that done unknowingly, that done in a dream, that not done intentionally, that which another person forced you to do against your will, that done only once and then regretted, [and so forth]
2. Accumulated, but not done. Tsongkhapa writes:
Killing that is karma that you have accumulated but have not done is seen in the following case: you investigate and analyze for a long time in order to kill a living being, but you do not kill it.
3. Done and accumulated. Tsongkhapa writes:
Killing that is karma that you have done and accumulated is seen in all the killing not included in the previous two permutations.
4. Killing that is karma that you have neither done nor accumulated is whatever is not included in the above three. Tsongkhapa does not give example.
He further explains that karma that is not done and accumulated (the third) is karma whose result you will not definitely experience.
The Levels of Yogic Deeds (Yogācārabhūmi-śāstra):
Karma whose result you will definitely experience is that consciously done and accumulated. Karma whose result you are not certain to experience is that consciously done but not accumulated.
Further debates settles on the meaning and reason of such a statement: karma that is done and not accumulated, for instance, is not as heavy as that which is done and accumulated. The weight of karma is one of the five factors that determines the time and the likelihood of its ripening.
- Killing breaks the first precept and planning does not.
- One commits Kaya(bodily) Kamma when killing. Planning may not necessarily involve Kaya Kamma as it could just be limited to thinking.
- Killing causes greater Kamma compared to simply planning as it takes a greater degree of corruption in the mind states to actually carry out the plan until it's fulfillment.
It is the teaching of the Jains (which is actually very accurate under the label of Buddhism also here, who held the view that physical actions have the highest impact) that physical deeds are heavier then mental. As for what the Buddha taught is that mental actions are by far more powerful and have much more consequences, even to an extend that can lead beyond all kamma of what simple verbal and physical actions are incapable to cause.
One should understand the difference between "breaking precepts" and act unskillful by thoughts, words and deeds.
In how far certain kamma (action by intend) is heavier does actually not depend only on whether it's "only" by thoughts, words (signs) or physical deeds, but foremost on the quality of mind, prompted, unprompted and the amount of effort put into to uphold such.
People would wonder about the huge impact of certain "silence and inaction" and the minimal impart of their "rites and rituals" with impure mind.
SO one should be clear that "just" mental approving, or mental joy, or agreement, or accept association mentally "earns" likewise fruits like if ordered or done by oneself. And that is why one should take care not to associate with fools, especially mental approve or acceptance.
And like always, before you up-vote and down-vote to make your earnings here, think and investigate twice, since it's the law of nature that mass and fools are in one line.
See also On denying Defilement and the Uposatha of the wolf is also a good lesson for some open to look at ones defilements.
Why one "planing" to kill "act"-ually does not may have many reasons, sometimes just verbal or physical incapacitates, and if such would protect and lead to liberation, all "we" would need to make "heaven on earth" would be cutting of all limbs and possibilities of signs of all beings...
As for what is breaking the precepts, and how such is punished in certain society, that is another topic, but also requires the intention to be investigated, at least in the society and convention of the Noble Ones.
(note that this here is not given for trade, exchange, stacks and entertainment here, but as a tiny door to escape for those capable)