If you didn't know he was convulsing and attacked him as a rager farther than 8 steps from the cabal, sure, that's courageous. As a shifter, I've had my fair share of blowback from ragers because I killed them when they were injured from another fight so I was a "coward". Well, unless I'm an eagle, I have no way of knowing what condition you're in, so as far as courage and cowardice goes, it has no bearing. But, killing you when you're convulsing has nothing to do with being a coward or brave as long as I'll attack you when you're healthy as well.

Also, courage means facing dangers on principle even when it might kill you. That does not mean you have to let a crippled mage walk away because it would be too easy to kill him. It also doesn't mean not using strategy to increase the odds of killing said mage to the maximum possible.

The oath to Tahren is to destroy all magic in Thera, not to make sure mages have an equal chance of killing you in every battle you fight.