Actually, the main problem with the theory is that other than the appearance of the same three-word-phrase, there's no other similarity between the DC sniper and the film "Malice".
"Malice" has no scene with tarot cards, the phrase is spoken and not written or left as a "message", and the phrase is pronounced as an arrogant statement of the character's ability to SAVE lives (as a doctor -- who is not a serial killer) and not in relation to the taking of lives.
And it's not that uncommon a phrase. You can find it in the film "Impromptu" ("Good evening. I am God"), "The Lawnmower Man" ("I am God here!"), the 1996 TV series "Superman" ("I am many things, Kal-El... here, I am God"), and so on.