Unfit to govern = executed?
I hate to see what a Rossiu-run planet would look like... Stalin would be a more palatable leader by comparison. Rossiu isn't any more fit to lead than the others, he is just more arrogant.
Yes, he IS a bastard for the sake of doing so. If Rossiu want to save more lives, he can just fake Simon's execution. That would be one extra life he would save, and the mob wouldn't know any different.
If Rossiu could hide Lord Genome's head in the basement, he could hide Simon. And yet, he kept Lord Genome alive but want Simon dead. The only explanation is that Rossiu is jealous of Simon and want him removed permanently.
Killing Simon is RISKY. He is the best fighter they've got. Thus to murder him for personal political gain is to put at risk the lives of everyone on the planet.