As already mentioned, empathy. You are referencing the age old argument of moral origins, you can simply google to find out many sides of the argument (example

As already mentioned, empathy. You are referencing the age old argument of moral origins, you can simply google to find out many sides of the argument (example: https://www.quora.com/If-not-for-religion-where-do-morals-and-values-come-from https://www.quora.com/If-not-for-religion-where-do-morals-an...). However, its quite simple to resolve. The gist of it is: if you need a deity holding judgement over you to distinguish right from wrong, you are probably a psychopath.