Betrayal is the act of treachery or a violation of a trust or allegiance. This can be to one's friends, country, or God.
Secretly the enemy
One form of betrayal is that they were the enemy the whole time and the friendship was based on his/her mission. In fiction this would either end with them changing their ways because the friendship meant something to them or they go through with the betrayal and the betrayed feel used.
Decided in an Instant
This version of betrayal indicates that in a short period of time the person decided to turn their back to their friends for their goals. When this happens the friends tend to feel remorseful, wondering if there was anything they could of done to prevent this, or knowing the fact that next time they cross paths one will probably die.
Just a trivial fact, but in the Divine Comedy the lowest level of hell was meant for those who betrayed their friends, country, or god. The area consisted on a frozen lake hosting several bodies (your betrayal determined how deep you are in the lake) with Satan in the middle.