Demons: Definition of a demon

                                                                  Q. In Bhagavad-Gita 11.20, Arjuna describes that he sees all the planetary systems perplexed before the universal form of the Lord.  In the purport, Srila Prabhupada states:

                                                                  Q. We hear that Kamsa had become the ruler of Mathura (where he made alliance with other demons, he married the two daughters of Jarasandha, he subdued or killed others) and his ambition was to be the Emperor even of all demigods.

                                                                  Also we read and hear about Kurus, Bhisma, Drona, Yudhisthira Maharaja etc.
                                                                  Many times they are also mentioned as emperor and ruler of planet earth.

                                                                  So,

                                                                  1. What's the correct understanding? Because both parties are contemporary and physical distance is very small (Delhi & Mathura).
                                                                  2. Is there any mention of Kamsa attacking Hastinapur? or