Killing an animagus (almost certainly) returns them to their human form.
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> *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!’*

> <suB><sup>Harry Potter and the Half Blood Prince</suB></sup>