What Latin phrase might be used for "method of murder" on the model of "modus operandi"?

Allowing for the fact that one doesn't necessarily just plug in words, I'd suggest modus caedis.

When I took Latin, our favorite verb for "to kill" was interficere because it's literally "to make/do between" - killing someone by slicing them up. Cannibals have a modus interfectionis.

Why not go for a parallel style and use the gerund (verbal noun)? Modus interficiendi.

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.

Ethod-may of Urder-may ?

What about "modus interfeci"? Is that ungrammatical? It gets about 22 Google-hits, all webpages in Latin; while "modus interficiendi" gets 0 G-hits (not that that necessarily means anything).

What about "modus interfeci"? Is that ungrammatical?

Well, it's hard to understand. Modus is the singular nominative form of a noun, and interfeci is the first person singular active present perfect form of a verb. It could mean something like: I, the manner, have killed (it), but it doesn't mean the manner of killing.

All the pages which Google finds are German and offline. Scanning at them they don't seem to make any sense in Latin. Most of the words are in their lemmatized dictionary citation form. It looks more like lorem ipsum greeked text used for some kind of page-ranking scheme.

[Minor typo correction and added link.]

Thanks for clearing that up.

modus interficiendi

One more thing: Is that a hard "c" (FEK-Y) or soft (FESH-Y)?

Depends on what kind Latin you're speaking. In Classical Latin it would be a K sound, in Italianate Church Latin it would be a CH sound, in German Church Latin a TS sound and in (God forbid) USn legal Latin the whole thing would probably be pronounced MOHD iss inter fih SIGN die.