How do we know exactly that a "no pain method" of killing is painless?
Because he / she / it wouldn't get a chance to scream, shout, weep, cry and twitch due to the fact that the nervous system is first shut off before vital organs are?
And: if so, would you do that to yourself?
It is not only the screaming, crying, suffering from pain, etc that show the fear of death, but that moment when "you are not in control and forced to leave your body into the unknown".
If a kid is disallowed by his parents to have chocolate after 6pm (regardless the reason), would then be OK if he melted the chocolate and make a hot cup of chocolate instead?
If not, why would painless killing not be a killing? Your "intention" (thought / sense) is unaltered - to kill, "different" only by a method with which you justify yourself.
It "is" not about whether or not a being would become human through re-birth by being the judge yourself, it is about you accumulating the "debt" and ending a life that has its "duties" and its destiny through re-births.
Let just make a very rough example that a cow was re-birthed from someone in the previous life that had done vicious crimes and deeds. So it, in this life, has to farm all day, subjected to torment such as being milked daily and forced to see its own newborns (calves) being taken away and slaughtered. And in the end, without getting a chance to "rest", it itself got slaughtered.
You come along and take this cow in, without making it to do all of the above and treat it pretty well until its time is up.
You would then be "clearing" its debt and at the same time you produce a good cycle.
That is all this is about and that is - Compassion. The fundamental.
It is hard to grasp for others, because they got so used to of thinking of "profit and margin" - I do something good, I earn a point in my Karma Bank with interest. I did something bad, I lost Karma points and thus the interest.
Compassion and Karma is unlike capitalism.
Everyone has it - Compassion, but it is "covered" by our senses (desires: taste, lust, selfishness, touches, perceptions), thoughts (judgements, clinging, biases etc).
And once one finally grasps it, one would try to benefit all beings without the "self" and without thinking of Karma before or after doing something beneficial to other beings.