Killing an animagus (almost certainly) returns them to their human form.
The harsh cry startled the fox, now crouching almost flat in the undergrowth. It leapt from its hiding place and up the bank. There was a flash of green light, a yelp, and the fox fell back to the ground, dead.
The second figure [Bellatrix] turned over the animal with its toe.
‘Just a fox,’ said a woman’s voice dismissively from under the hood. ‘I thought perhaps an Auror – Cissy, wait!’
Harry Potter and the Half Blood Prince