These shoes are horribly uncomfortable but they look nice so they need to be destroyed! Ideas please!

Do you want to destroy them while you are wearing them, or with them off?

While wearing:
Fill with custard and put back on.
Prepare a 6" deep tank of oil, step into it.

While not wearing:
Attack with tools, use pliers to cut and break off the heels, use snips to cut the sides down.
Place in a suitable fireproof metal tray, spray down with flamable liquid, set fire to them.

Filled with emulsion paint. You are good at that.

Fill a bucket with slop and while wearing stockings with these sexy shoes on. Plunge them in then pie yourself in the face repeatedly! Something I've done many times myself. Although my shoes do tend to survive with a bit of cleaning. The messy thing that really ruins heels tends to be going on muddy walks

I love this pair of shoes but they give me cramp when trying to wear them...

Sort of as Portia suggests.... filled with oatmeal and squished on.

raw eggs

Fill them with pva glue

Dairy products, oil-based paint, glue. Not cleaning the heels afterwards because the mould growing will destroy them.

Destroyed a pair of heels with oil-based paint.