Yes Ranger is pretty much the best damage dealer with bows, not that you wont be able to make damage with the assassin using bows, but you wont get the key abilities that rangers get.
Your key abilities as an assassin will be for melee combat so using ranged weapon will only make you do less damage then your full potential.
Betraying aint that bad really, often enough other people are betraying so you can team and do some of the killing together, if your lucky you can find someone who already betrayed in the past and have some experience to help you.