"i just wanted to see whats your argument for claiming that murder is morally wrong"
You want a meta-ethical explanation, not just an ethical one.
Ethics is the study of how one should behave, or in other words, right and wrong.
Meta-Ethics is the study of what makes something ethical.
For example, saying that God determines what is right and wrong is not an ethical conversation, but a Meta-Ethical conversation, as the core of the conversation would be in determining the nature of ethics.
Another example of Meta-Ethical discussion would be when you see people say something like: There is no right and wrong. Anything goes.
This is a meta-ethical definition that supposes that there is no objective moral truth.
Within that conversation then, you could have a conversation about ethics.
It would look like this...
MetaEthical foundation: There is no objective moral truth, therefore anything goes, as there is no right or wrong.
Ethical Dilemma: Since nothing is right or wrong, how should we behave within this moral framework? We should let reason guide us to make logical choices.
So, in response to your question, my answer would be:
There is no objective moral truth. Applied ethics in this world should be a case of obeying reason.
Therefore, murder is wrong because of the categorical imperative.