But anywhere in canon, does she ever kill a mortal?
She decapitates Medusa in a baseball court, but she's not really a mortal anymore:
The fight takes place in a baseball court and is by Ares transmitted to the world. The fight is brutal, as Diana fights blind, first by a piece of cloth, then by snake venom she pours into her own eyes. But Diana is victorious in the end as once again, Medusa is decapitated.
Some examples of WW almost killing mortals:
Once when she believes Superman has been killed:
And once when some bank robbers interrupt her day off: