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 a faction of the Culture, trying to prevent the Culture as a whole becoming complacent and decadent.
  • Though the latter seems unlikely as insiders should have been aware of the limitations of the Culture's mind-scanning taboo.
    • Which could have been the point. A faction of the Culture might not want another genocide 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.
        • Remember that not only did the attack have no chance of succeeding vs. an alerted target, but the casualty count would have come from the unplanned disruption of services from Masaq' Hub... but Masaq had already shifted all Orbital housekeeping to automated systems in preparation for its own suicide. So there's no civilian casualties to worry about.

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?
• The same reason the US only executes criminal defendants after convicting them in a fair trial with due process of law, but sent Navy SEALs to kill Osama bin Laden in his house while he was sleeping? The people who are fighting wars with you don't get treated by the same rules as your own citizens in peacetime. That's kinda part of the definition of "war", the breakdown of the normal social order. And the people who 'pissed them off' in this instance were the masterminds of a major act of war vs. the Culture and so got the Osama treatment.