Stabbed in the back

I've made lots of friends in my life, and met lots of people in the world who are trustful, friendly and honest.
Although there are the black soup of all people relationships, the double-dealres who are friendly and say that they are honest with you, but they're the ones also who appear first to "stab" you in the back, if the oppotunity turns up. I've gained a personal experience like this while being in secondary school. At that time I was still the person who thought that all people are good and no one wants to do any harm, but this wolrd view of mine had suddenly changed when people in the class accused that I simulate my migraine to get a chance not to write the tests.
This so called "problem" had even been a separate topic in a lesson. Forunately there were some people who supperted me, and I'm going to bee always thankful for them for this, but the majority was against me, and here comes the point of my title, the person who knew me since kindergarten and saw/knew the history of my illness, who knew what my migraine goes with, was in the first line to "attack", and this way getting me "stabbed in the back".