Thanks for the question.
I will provide justification.
Point 1 - Killing adults is immoral
This claim is founded upon ethics. Different ethical systems generally fall into two main categories:
- Religion. For example, Christianity, which inspired human rights in the first place. 
- Philosophy. For example Kant, Utilitarianism and many others. Among them you also have one  or my "basic" version 
"all men are created equal - and are bestowed by their creator certain unalienable rights" This was written in a country where everyone was at least culturally Christian.
P1: Humans value their own well-being
P2: If you desire others to respect your well-being you ought to respect theirs
Con: Therefore you ought to value well-being
P1: Adults might want to kill each other
P2: Adults does not want to be killed
C: Adults writes a contract, where adults are prohibited from killing each other
Extend the line of though until we have a functional ethic's system. Which one does not matter since all must forbid the killing of adults.
Point 1 has been justified beyond a reasonable doubt.
Any other objection?