I have understood from tibetan teachers that kamma is indeed related to how effective one deeds are. One cannot become a heir of the kamma of killing, for example, while one has never really killed someone. Mere thinking about killing, does not make one a heir of the kamma of killing.
I believe this makes sense. But the longer mind is occupied with these thoughts of killing, and even making preperations to do so, is not at all innocent ofcourse. It is like collecting a kamma seed that becomes more and more powerful, loaded. Kamma seeds are loaded mano-vinnana’s and those are fed by constant attention for killing (stealing, lying, harming etc). It is like a killing vinnana grows. It becomes more and more concrete, manifest. It becomes ones inner world.
I have understood that the most potential has that kamma seed that contains all aspects of a full kammic Path of killing: intention, identifying another living being that must be killed, no regret, no shame, no fear for consequences, preperations for killing, execution of plan, and really killing someone. Now one is owner of the full kammic path of killing. This is a collected kamma seed with a great potential. It does not need much to ripen. Now one is the heir of the kamma of killing.
If a thought of killing immediately disappears, to me it makes no sense to think one is now the owner or heir of the kamma of killing.
Also, unintentionally killing a being is also not seen as becoming a heir of the kamma of killing.
One cannot become an heir of kamma without intention to steal, kill, lie, etc.
The kammic results seem to become more and more serious for oneself if there is a complete kammic Path involved.
Sometimes also there is distinction made between kamma that is created and collected (become a heir of that kamma). Alice has created the kamma of harsh speech, but did not collect it, because the harsh speech was not received by Bob.
Unintentional killing means that the kamma of killing is created, there is really a being killed, but it was not intentional, and that person has not collected that kamma of killing.