Why are my cats acting like such bastards?

I took in a stray cat a couple of weeks ago, and the other five cats I have hissed at him, of course. Now that they're finished with the hissing, they're being passive aggressive and they're crapping in the litter and not covering it up. So, they lay down the turds and it's like they're saying "screw you and your new cat." When will they stop this?