User claims hack behind record 83 BTC transaction fee

The same account also produced a cryptographic signature to prove they had the key behind the transaction. This indicates whoever is behind the user account had the key in question, but it's possible a bad actor had access as well.

Bitcoin user claims to be victim of hack which led to record $3 million transaction fee
 https://www.theblock.co/post/264744/bitcoin-hack-victim-record-3-million-transaction-fee

Instances such as this are why it's important to generate fresh keys and ensure devices have not been tampered with.