The Culture/Headscratchers

Chelgarian Allies of Look to Windward.
Was it ever revealed/implied who were the other Involveds behind the plot?
 * No, it wasn't. The Masaq' Orbital Mind even suggested it might be
 * Though the latter seems unlikely
 * Which could have been the point.  might not want   but still want to make a threatening point about what The Culture had done.
 * Seems far too costly. The message would have still been heard with a hundred times less civilian casualties.

How come The Culture spends so much time informing the reader how "capital punishment is bad, mkay?" -- but are perfectly willing to send a nanotech assassin to gorily murder people who piss them off?
 * Because the Minds who lead The Culture speak softly and prefer the use of a covert yet unilaterally psychotic branch of Contact to a big, war-starting stick?