Cats were desert animals. They evolved to have extremely efficient kidneys and maintain water within the body. Their body temperature is also higher than humans. This is why many domestic cats seldom drink water (they get most of their water from their food, unless it's dry food) and why they love hot places (by windows, near radiators, etc).

So, the notion that cats are getting aggressive because of the heat doesn't match what we know about their evolution and physiology. It's pretty hot on the African plains and yet lions quite happily lounge around for most of the day. Cats only get aggressive, like most mammals, when territory is encroached upon, they're being continually annoyed, or they're off hunting.