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body count is not the only factor to consider. There are other consequences.
Yes, under the assumption that all lives involved are equal. The decision to not kill the one person is the same decision that leads to the death of a thousand. Doing nothing doesn't relieve you of moral responsibility. It is a decision just like doing something is.
What makes their life less valuable than other people's lives?
Moral varies according to the culture and values of a group of people, therefore you can't make any assumption regarding the subject unless you place it in an appropriate context.
This depends on the 1 person getting killed and the 1000 people getting killed.
If either of these groups are bad people, then it changes a lot.
It is justified murdering 50 people to feed other 100? What is the numerical limit to define them as acceptable or not?
Someone with deontological ethics might not accept that, because the outcome should not govern the decision.
it is not epistemologically possible to know that killing 1 person now will directly save 1000 in the future outside of a contrived hypothetical scenario where we are assumed to have perfect knowledge. If implemented as policy it could promote ignorance of unintended consequences and greater death.
There is no built in value for life, morality is something one comes up to protect oneself. Accordingly to reasons, situation and outcome, killing anyone to either save or not save is completely justifiable.
1 person will die instead of 1000
By killing that 1 person you drag him into the equation and end his life. If you would just act passively, 1000 would die, and that 1 person would have never had to be killed by yourself.