Of course he knows that he did the wrong thing, everyone makes mistakes no matter how perfect they try to act. Deciding to have Simon be executed was a short-sighted decision in reaction to how unruly the mobs were getting. He looked absolutely horrified when they started getting violent and he ended up getting a rock chucked at his head. I don't think it was personal, it's just that using Simon as the scapegoat seemed like the easiest thing to do at the time. He was raised in a village where people were cast out into surface world to die so that the rest of the village might live. One would think that that would affect a person's better judgment.
Also, I forgot, was his execution to be public or private? If public, Rossiu would pretty much have to execute him as people were expecting to see him die. If not, who's to say that he was actually going to kill him? I see no reason for Rossiu's choice to be personally driven. Simon is in no position to threaten Rossiu in the first place as he is hated by pretty much every citizen.
And you're right, Rossiu is not truly cold and calculating, he just tries to be. But now that his plans are backfiring and almost everyone on he planet is endangered, he's losing it. As hard as he tries to be an emotionless thinker, he does have emotions and he's beginning to regret what he's done. His plans are too short-sighted. Yes, killing Simon might endanger the people, but I really don't think he was thinking about that at the time. Before he learned that the entire surface would be wiped out and not even people in the underground would survive, he probably thought that he could satisfy the people's blood lust, evacuate the people living on the surface, and then deal with the Anti-Spiral after the moon crashed into the Earth.
Then again, maybe you know something that I don't? I don't know about you, but this is my first time watching the series. If it turns out that you're right and Rossiu is an evil monster that wants Simon dead so that no one can stand in his way of total control, then fine. I'm just taking what I know about the show and the character of Rossiu himself as shown so far and speculating. In the past he was shown to be a good person, if not a little timid and stuffy while coming from a rather cold and flawed society. I see no reason that, without explanation, he suddenly became evil, power-hungry, and malicious within the seven years he spent with the others.
Don't get me wrong, I don't approve of what he's doing at all. I just believe that he's simply doing things in the most efficient way he knows. I look down on the way he's currently handling things, but I do not hate him and I'm elaborating on why. You also have to take into account that these people are all still very young. I don't expect a 21-year-old, someone only a year older than me, to not screw up big time and make some really stupid mistakes when to comes to handling government and civil matters.