Cherry if it smells bad, I recommend you empty the butt, scrape/brush out the inside surfaces, thoroughly, then rinse, and then refill with some tap water unless it rains.
Water butts that are not regularly cleaned (say every 3 months), unless the lid is tight fitting to exclude light, will grow all sorts of life forms. Some it appears, can harm seedlings.
A trial by Gardening Which? showed better growth of pot plants from tap water than from butt water.
I clean my butts every month in the summer, water seedlings from them, am careful to keep the bottom of cans clean when refilling.
