Smelly/stagnant compost - bad for plants?

[SquishySquirmy]

I usually reuse old compost, but some of last years was left in a (large) plant pot over the winter. It didn't drain properly, and so went all horrible and stagnant (with a small puddle of water sitting on the top).
I've been delaying dealing with it because it is somewhat out of site out of mind behind the shed. When disturbed in any way, it smells AWFUL. It needs to go somewhere it can drain.
If I put it on my plants, will it be bad for them?
Would I be ok to use it for pot plants?
Could I spread it around my flowerbeds as a (smelly) mulch?
Or is it a lost cause?

I wouldn't use it as it is. Wet and smelly is usually a sign that it's too acid. If you can mix it up with some dry compost, and even better if you can also add in some wood ash, or other source of lime, it should balance things out and make it more useable.

It's probably too full of nitrogen. Can you add a bunch of brown matter like paper and especially cardboard to balance it out? Compost shouldn't be smelly and you might do more harm than good putting it on your plants.

I'd add it to the compost heap, I wouldn't use it on plants now.