Why Is The Sun Considered A Malefic In Vedic Astrology?

Although the Sun or Surya is a deva (godly), satwic and dharmic graha, it burns the house, sign and nakshatra where it sits.

Legendary astrologer Sanjay Rath sir had also mentioned this in his video on YT with KRS. That the nakshatra where sun is located is slightly burnt. This burning is symbolic not literal... ofcourse. Sun represents sacrifice. 

Lord Ram who was a Suryavanshi king (lineage of Surya) had to make a lot of sacrifices for his subjects on the expense of his personal happiness. Sun is highly moralistic graha, so it will makes one sacrifice due to ethics and principles. 

Sun is highly spiritual planet as it is natural atmakaraka, it cares more about the spiritual growth of the person than the materialistic growth hence it is considered bad in terms of materialistic existence.

As sun is associated with Agni tatwa or fire element, the fire burns away the impurities or the material part but the spirit lives on. As fire can't burn that which is permanent and the spirit is permanent.