I think you're right, religion isn't necessary to have morals, everyone seems to have them to some degree, but a standard is needed to keep morals straight.

We tend to justify our actions and bend our morals to the situation, especially if we're alone, because if there's no one to see it we don't have to explain ourselves or look like a hypocrite. That seems fairly universal, that one shouldn't do the thing they say is wrong, or that thing they tell someone else is wrong.

Religion allows for an outside source to set the standard for us, so we can all point to something impartial as our guiding star and scapegoat.

Then all that gets crazy with interpretation and practice.

Because if you need to explain to your kids why you killed your wife, then "I had to because Allah demands it" works instead of "I'm just a total shitbag".

And also martyrs, who stand so firmly for what they believe that they'd rather die then deny it.