In this question you have presented men as helpers or saviors. Here I am not trying to answer this question directly; but some clues for thought.
Who have more right to kill a cow, to men or to carnivorous animals?
If I state, that if nobody ate cows, the cow species would be worse
Happiness is personal. In this world there are many millions of people living in serious conditions. Do we need to kill them all?
If you kill a cow, are you a savior or terminator? If you are doing the job of a terminator, would it be a good act?
I theorize, that in general, being consumed by humans is the best
evolutionary step a species can make.
Are the human beings the saviors of all other creatures of this earth? What is special about the cow? Haven't you seen animals that depend on cows? Then what are you actually doing to those animals? If so, how could you regard it as the best evolutionary step?
Since the cow is a big domestic animal, we see its sufferings and give more importance to them only. Are we aware of the sufferings of wild animals? What would be the reason for the invasion (even though I don't wish to call so) of some wild animals (to populated areas)? If your arguments are rational you should eat them also.
If agents of pollination like bees, beetles, butterflies became extinct what would happen to these saviors (human beings)? Is this impossible? Don't we get its warnings every now and then? What would happen to the cow population consequently? Then, without being eaten by any man/animal who would control the cow population?
We know only very few relations between animals and other living things.
All the natural processes are happening even in its micro level. So "If nobody did it" is not a question for discussion. Nature can reduce the cattle population if it needs that.
So "If nobody did it" (in this case) is a fallacy.
I want to distinguish between killing and bad treatment
Some bad treatments are worse than killing. By bad treatment you are allowing the victim to suffer pain for a (comparatively) long period.
In case of bad treatment, you can apologize to the victim. Or if it is a creature, you can do remedies for that. But this cannot be done in the case of killing.
One 'can' treat another badly many times. But killing; one can do it once only.
[I think this would be enough for this question]