Uhm i noticed that if you play an evil type of character you get easier better weapons is a good knight useless?
Your characters can change their alignment during the game. If you constantly attack monsters when encountering them you will slowly shift your alignment towards evil, but if you offer bribes instead you'll shift towards good. There are certain items that only work for good, evil or neutral characters. I don't think there is a distinct advantage to one alignment over another.
You can restore your alignment at a temple, or with the restore alignment spell. If you get your good knight to be evil, then give him an evil-aligned weapon, then restore his alignment, he can keep using the weapon as long as you don't un-equip it.