Yes, that was an influence. :) But the name of the murderer doesn’t really answer the question. "Кто стоит за...?" is digging deeper, so to say; it asks about people interested in something (usually something bad, like in this example), those who are the real cause of something happening.
Since Allahu akbar is, as far as I know, not a comparative form, it is simply "Бог велик" or "Аллах велик" (in Islamic contexts, "God" is nearly always "Аллах"). But this phrase is well-known, so "Аллах акбар!" will also be understood by everyone.
About the comparative of великий: it can only be более велик, великее doesn’t exist.
By the way, "Аллах ве́лик", with the stress on the first syllable, means "Allah is a bicycle" (ве́лик being a slangy word for this means of transport) — a pun said to have offended some people.