Bees without a nest??

For the last few weeks we have had semi agressive bees in the backyard. The gather at open windows and try to get in the house from spaces between screen doors. If you are outside with them they get curious to the point of harassing you. They will give chase for a moment or so if you run from them and then it's back to the doors and windows. My husband had to kill one, I'm sorry I know we love bees, bit it was extremely aggressive. Ever since that night he killed on they come back a lot, nearly everyday. Question is, why are they trying to come in my house during all times of the day? We are in southern California, the weather is in the 90s right now if that helps. We dont have a hive in our house, on the outside or in our yard. These bees are new to us, maybe a month.
Any thoughts would help us bee novices, we just dont want to harm them or them sting anyone