Question: how could someone you trust stab you in the back? 

Answer: Trust is just an emotion. People without ethics don't care much about your emotions but mostly about their own self-interest and advancement at any cost. Actually most of us are like that in one way or another. When it comes to what's really important in your life, sometimes you will forsake friendship and loyalty. I guess it's not true friendship or loyalty.
This question from the Yahoo Q&A forum is categorized in the topic: Health