A friend's loyalty is tested when times are not so good.
A true friend will be loyal to you even when she disagrees with you.
He/she will talk to you in private about personal opinions concerning what you are doing, but in public, she will maintain her loyalty, even if you are dead wrong. She will never trash you in public, period!
Being a loyal friend does not necessarily mean you agree with everything your friend does, but it does mean that you are dedicated to your friend - right or wrong.
True friendship is not conditional.
I have a few friends that I will trust my life with, because they are proven with blood, honour, and integrity. I am abundantly lucky......most people don't even have one.
And you are also right that everyone needs to consider what kind of friend they are to others...if you aren't trustworthy yourself, why would you expect someone else to be a loyal, trustworthy friend to you?
But if you live your life by high standards, being trustworthy and a true friend to your real friends should be a given.