For the Imperial Legion quest "Suryn Athones' Slanders," I'm supposed to kill him. However, I literally cannot attack him, even in self-defense, without being reported for assault and expelled from the Temple. Even if I taunt him into attacking first, retaliation is reported as assault.

Is there any way to fix this?

There is no private place to lure him to, though. He's in the Justice Offices, which are basically three small rooms (all occupied by other Ordinators) connected by a short, narrow hallway. Even locking the doors with Fenrick's Doorjam isn't private enough to prevent me from being reported.

I know you mentioned before that it didn't work well for you, but looks like "Taunt" did the job for this person:

https://www.youtube.com/watch?v=jVbk1hKP4CI

I wish I knew what was different about my game. I'm not supposed to be reported and expelled for attacking someone who attacked me first, but somehow I am.

A combination of Frenzy and taunting seemed to do the trick. I cast Frenzying Touch on him, which wasn't enough to make him attack me, but then I taunted him while he was under the spell, and when he attacked me and I retaliated, I didn't get a bounty or an expulsion.

Glad it all worked out :)