We had this happen in Toronto 2 years ago with our Sumatrans. Hari, who oddly enough came to us from San Diego, killed Brynthe our very successful experienced female. They were being introduced and all seemed to be going well when Hari attacked her and crushed her wind pipe killing her instantly. Nothing keepers could do. Right now they are trying to introduce him to a second female, Kemala, but are being very cautious.
We also had a male Amur, Tonghua, who killed his first mate about a decade ago. He went on to successfully breed a number of times with the first females replacement, Tatianna.
I remember being told back then that cats can be very tricky to introduce. Sometimes its easy. Sometimes they get into fights. Sometimes it results in the females death.