Why are we not allowed to kill

[naut]

I couldn't think of valid reason why killing is bad.

There isn't any.

Society tries to create intricate moral codes and methodologies and philosophies in an attempt to fabricate reasons why it is, such as Good vs Evil, Truth vs Lies, Moralism, Human Equality and Humanism. But, all of these are just thoughts, nothing more.

Also there is the matter of whether you believe in good and bad as universal truths. But, I digress before I start arguing semantics.

Edit: Adrian beat me to it.