I think it has something to do with their weirdo ritual; if they slaughter everything it 'does not count' for summoning...Err, Mister Twister. They have to deliberately space out the killings by some unknowable amount of time. Combine this with the difficulty of getting off such secretive, hidden kills that easily-secret powers or not-and just isolating someone for the kill and impending mutilation thereafter, I think they need time to plot out their marks. Given the warning, however, I'm fairly sure there's some caveat based on 'innocents killing innocents,' or somesuch.

As for letting them catch on, I think Mew gave us the power to search places undetected. They may not even realize what we took is gone...Ugh. I don't know-but there's already killers loose, and I don't think sitting around doing nothing or 'waiting for the right time' will work, especially if that box has something in it that could help us.

I'm just theorizing over here.