Is it because we cause a being to suffer and experience pain while it's/he's being killed?
Or, is it because we put an end to its/his relationships and desires?
Or what reason can it be?
It is our Perception of the Sensation / Feeling experienced when result bears fruit is what distinguished the Karma as bad and not. I.e., Bad Karma is what you Perceive the results as undesirable and Good Karma is what we would perceive the results as favourable when the doer experiences the result, as well as skill full when this is conducive to reach the Buddhist Goal of Nirvana and un skilful when it is un conducive in reaching the final goal.
Say you kill you will experience the same when your actions catcher upto you. Hence what is needed is to treat other like you would be treated, or treatments you would evaluate positively if you experience it.
Also there is skilfulness of your action towards the goal of liberation. In this light any action which creates fabrication which hamper concentration and insight or the opportunity to practice can be considered unwholesome. Killing has aversion and / or craving as base. This creates fabrication which give painful sensation when meditating which will hamper concentration, insight as well as the needed longevity and conducive human birth to practice Vipassana hence such actions are un skilful.
For a worldly person (or someone not very deep and intensive in Vipassana) it is helpful to build-up Perception (Sanna) what these are good action and bad action so he is steered away from un skilful / un wholesome action towards skilful / wholesome action. As Perception clouds your sense of reality such mental maps ultimately have to be dissolved away too.
Karma is generated by intention, according to the Buddha, in contradistinction to Jainism, which identifies the cause of karma with action. Although all karma represents bondage to samsara, some karma generates more positive results and some karma generates more negative results, including the karma of liberation itself.
All experiences reflect the quality of the karma that one engenders. Killing originates in states of desire, passion, agitation, anger, and unhappiness, and thus those states return to one who has the intention to kill.
The proximate cause of killing is always hatred accompanied by delusion. But acts of killing can originate from all three evil roots. Greed accompanied by delusion can be the motivating factor in cases of killing to gain material benefits. Killing motivated by delusion is seen in cases of animal sacrifices done out of wrong views and killing the followers of other religions thinking it is a religious duty.
So intentional killing is bad karma as any thought or a thought process tainted by greed, hatred and delusion is bad Karma by definition.
Killing causes bad karma because of these reasons....
According to lord Buddha there must be Dvesha (a state of mind that feeds emotions like anger, confusion, hate) in a being's mind, to kill another or to harm another.
So killing or hurting someone crates bad karma because of internal reasons, not external reasons.
For example imagine you kill a baby who is yet to be born and who is yet to mature a proper body.
This baby has no relatives or relationships yet, it's not even a proper baby yet, and we are not sure if it can feel anything yet. But if you kill this baby it will cause bad karma to you: but why?
Here is the reason....
The "Dvesha" I mentioned above is one of the three sources of bad karma.
These are the three sources of bad karma:
So any verbal, mental or physical action that could cause these three states of mind is going to create bad karma.
These three can completely divert one's path towards nirvana and make him or her wrong do things. They pollute the mind of a person and diminish the potential they have to reach nirvana and to do anything in their lives.