It’s not that Kivas needed killing, it’s that data values life and would not kill to free himself, but will kill Kivas to prevent him from murdering others. Once Kivas was in custody Data had no interest in harming him.

So, the lesson would be that it is not virtuous to stand by and do nothing while some rich asshole is killing people for his own petty reasons, and that the logical course of action if no alternative exists is to shoot the fucker.