From the information we know.
The suspect committed felony assault and took the Officer's taser which would enable him to use it against the Officer and then take his firearm.
That is how he would be in fear of his life from the suspect.
[...]
As already provided in the other thread from the SCOTUS case of Tennessee v. Garner.
We conclude that such force may not be used unless it is necessary to prevent the escape and the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others.
As soon as the suspect took the taser he became a significant threat of serious physical injury to not just the Officer, but to others also if he was allowed to flee with it.
If the Officer did not see the suspect throw the taser he is still operating under a reasonable impression that the suspect still had it.