Occasionally a bought pot of basil will start to get blackened leaves after only a few days for some reason. The plants are crowded together in the pot and it isn't unknown for rot to set in at the stem bases. They're not supposed to be long-term plants. You can't beat basil from the garden, but you have to have the right climate. I've never had a cat-wee smell from basil, though I've had it occasionally from potted begonias, and I have horrible mildew on a gerbera just now. Stuff happens. Try again!