Naruto's RasenShuriken can usually kill average shinobi in one shot. If it's so powerful shouldn't it be forbidden?
It's not typical of strong techniques to be forbidden. Forbidden jutsu are often techniques that either damage the user, require a sacrifice (other than just Chakra), or is generally immoral.
Incidentally, Tsunade did declare Fuuton: Rasenshuriken as a forbidden jutsu after seeing the damage it caused Naruto's arm. Whether or not it's still forbidden after Naruto perfected it and managed to throw it (thus, nullifying the damage to himself).
Other examples of forbidden jutsu:
- Izanagi - Because it sacrifices an eye.
- Tajuu Kage Bunshin (Massive Shadow Clone) - Because the user might die from chakra loss
- Edo Tensei - Because of living sacrifice and immorality.
It is a forbidden jutsu. Later on Naruto learns to use Sage Art: Rasenshuriken which lets him throw it by applying nature chakra to maintain it's shape, but I think this is classified separately. Outside that, he can use chakra arms of his nine tails chakra mode to guide them, also negating the damage done to him. But, anyone who doesn't know senjutus or has a tail beast chakra mode will still be at risk, since it then has to be used as a melee jutsu.
It was originally classified as a forbiden jutsu because of the damage it can cause to the user.
"The name is a bit of a misnomer. The jutsu works like a poison. It causes damage to the cells that produce chakra. Even those of the user. If Naruto continues to use that ninjutsu he will lose the ability to mold chakra." - Fith Hokage, Sanin, Slug sage, Medical Sage Tsunade Senju
But when he throws it, it nullifies the damage to himself. So I suppose it was deforbiden.