The judicial phrase inter sīcāriōs means ‘on the charge of assassination.’ I’m not sure if this is as general use of inter (neither A&G nor L&S seems to say) but if I may title the ‘criminal inter‘ (from crīmen, charge/accusation) preposition, then consider the following possibilities:
- inter impudentēs: on the charge of shamelessness
- inter cinaedōs: on the charge of sodomy
By any and all means, correct me if I’m crazy or defend me if you think I might be on to something. I realize this is speculation; we only have limited textual data to support a theory on either side—it’s really a matter of personal judgment and extrapolation based on our available resources.
The Essential AG: 353.2
Latin for Addicts 
This is a good point to know, but I doubt one can generalize from it. First, so far I have found it only in Cicero; other authors prefer ‘de’. Second, even in Cicero, other crimes are reported with ‘de’. Here’s an example where he switches from inter to de in the same phrase.
[90] “quis ibi non est volneratus ferro Phrygio?” non necesse est omnis commemorare Curtios, Marios, denique Memmios quos iam aetas a proeliis avocabat, postremo Priamum ipsum senem, Antistium quem non modo aetas sed etiam leges pugnare prohibebant. iam quos nemo propter ignobilitatem nominat, sescenti sunt *qui inter sicarios et de veneficiis accusabant*; qui omnes, quod ad me attinet, vellem viverent. nihil enim mali est canes ibi quam plurimos esse ubi permulti observandi multaque servanda sunt. (Pro Sexto Roscio)
Here’s my wild, completely lacking in empirical support, theory. There was originally a phrase that meant something like, “convicted as a member of a group of assassins (or band of murderers).” It stuck. I’ve been reading Isidore’s Etymologies, though, so don’t believe a word I say.
All bets are lacking empirical support at this point, and I’ve been meaning to get around to the Etymologies. I think they’d be a good sequel to my current adventures with the Lives.