I hate to say this, but I think they were going for Zero Requiem 2.0. a.k.a "let's put the whole blame on one person to end the war for good and then kill him off" minus the part where Slaine's "death" was fake. That's my guess, but it'd be so stupid if it's true. Like you said, Saazbaum started the plot to kill Asseylum, not Slaine.