Why my cats hate each other

After continuous cat ownership since age 10, I am developing a theory that it's a mistake to have more than one cat. I believe this because cats like to rule their territory, and every time they see another cat, they feel it is a challenge to their supremacy. One of the worst things that hurts their feelings is the insult -- the absolute humiliation -- of being forced to share the love and attention of their owners. Purrcilla we have had the longest, and when David's cats, Schlepp and Schmuck, died, we ventured to the humane society and replaced them with two new cats. Purrcilla was furious, as she felt that her home was finally really "hers" without having to share it with other cats. One of the new cats Suki, has taking a liking to hanging out in Grandma's apartment, and will swat at Purrcilla if she sees Grandma petting Purrcilla! Mystic, our black Persian, loves to slowly creep up to Purrcilla, crowding her space and trapping her in various spots around the house until she hisses and tries to get past him. He will keep this up for hours, following her through the house. Purcilla's solution is to stand by the front door and turn her head to look plaintively at us, as if begging to go outside. Poor Purrcilla. Once out, she will stay sometimes stay out for days on end when the weather is warm, just to get away from "the freaks" (other two cats). Suki is just plain wigged out by any amount of activity, feline or human, and will actually hiss at us if we are moving around too much and she feels she is trapped. Late at night, as David and I are winding down for bed, she will start to whine in a strange cackling way, as if to tell us, "Go to bed already, and snuggle with me!" Despite their personality quirks, after 16 months together they have created some semblance of peace, staked out their personal territories for napping and hanging out, and usually do a pretty good job of pretending that the others don't exist.