Because it's been about 24 years and the noun was in the dictionary and the gerund wasn't and I could vaguely recall that facere is irregular and there's something weird about oper...

Now I've done my homework, I was confusing facere with its irregular passive cousin fieri and operari is a deponent verb. It turns out neither of those those things matter in forming the gerund so, yes, I agree "modus interficiendi" is better.