TRUST: When do you truly earn it?

It's so easy to lose trust in a friend. Having them earn it again can prove to be very difficult. I had a friend whom I trusted. I wouldn't quite say wholeheartedly, granted this friend wasn't someone I grew up with, nor was it someone I spent countless hours talking with, but nonetheless a friend, or so I thought. This person was someone who had a number of friends in common with me. In fact, I met this person through other friends.
I guess to cut to the chase, this person I considered a friend, blatantly let me down in the worst way. Then again, this person I thought was a friend was only a kid, and still is. Maybe it was my fault for thinking someone that young could make mature decisions, but I usually compare people to myself at that age, and I understood the difference between right and wrong. In fact, I was probably more of an adult at that age than I am now. So I tried to put aside this solecism, but the more time that passed, the more I thought about it, and the more it impaired my perspective of this person. We still have common friends, so just the thought of running into this person in public makes me want to punch them in the throat, but of course I wouldn't because I'm the bigger person, and any amount of thought I had put into this has been a waste of energy on my part.
I can't say I hate this person, whom I considered a friend, but I definitely would not accept this person back into my life. That ship has sailed. And although this person apologized (and it was a half-assed apology by the way....no feeling or remorse behind it, which really helped me see the worthlessness in the friendship we had) for their actions, or as I prefer to call it, "their mistake," I still find it very difficult to believe that someone could act so conniving, and then still want to be friends with me, and act as if they had done nothing wrong.
I think it pretty obvious that this person was not my friend long enough to understand that I don't work that way. This is the reason I find it so hard to trust people sometimes. It just takes one bad egg, I guess, to ruin it for the others. Let's just say this friend is not on my Facebook. As my friends very well know, I don't delete people from Facebook, no matter how much I disagree with something said, or someone's beliefs. I truly believe that everyone has a right to their opinion. This is why we have debates, people! But no matter how much I disagree with you, I would never delete you. It's simply my opinion that if I've friended you, I shouldn't unfriend (unless you're personally attacking me or stalking me). So I happened to look on my page today and noticed this "ex-friend" had deleted me. Well more power to this person for understanding that our friendship was not what is used to be, because whether or not I still liked this person, I wasn't going to delete them from my page, so I guess I do have to commend their actions for taking me off their page.
So what is it you should take away with you after reading this blog? If you're a friend of mine, please don't screw me over. The next blog may just be about you. I don't fight dirty....or at least too dirty. My weapons of choice are sarcasm and wit. My word is all I need because even that can sometimes hit below the belt.