I mean, you could have just started off by asking, rather than taking the crusader route of asserting that your faith is objective truth and then making us come to you.
But, for your answer, murder is wrong for the same reason rape and theft (in most cases) are wrong; you're taking something (in this case, life) from someone else without their consent. If you want to think of the Ten Commandments as objective moral law, I'd have no qualms with that, but you have to understand that they are also Jewish in origin, and the Commandments themselves are borrowed from a condensed version of Hammurabi's code; which is to say, the idea of Supreme Wrongs has existed LONG before Christianity
It's also worth mentioning that I was raised Christian, and I still occasionally go to and appreciated church -- but I was also religious to an extreme at one point, and after seeing what that did to me, I've distanced myself considerably from Christianity and prefer a more "do good without witness or reward" approach
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? God's a capitalist?