Declawing in cats is a controversial topic. When carried out properly, by a qualified veterinarian, it is performed under anesthetic with pain relief during and after the procedure. However, even when performed correctly, it has been associated with chronic pain and behavioral issues such as aggression.
Many cat experts consider the procedure inhumane and unethical, and it is actually banned in many parts of the world. Thankfully there are lots of good alternatives to declawing that are much more humane.