In Sc1 it is also reasonable to ask whether Mengsk's seemingly 'out-of character' rant really is proper evidence for the case of him being a "totally selfish and monomaniacal person from the start" since it is only one incident, appears last (therefore being memorable), could be misconstrued and possibly hearsay. And yes, I know that last sentence is bullcrap and apologetic to Mengsk given what we know about him
now, but in order to wrap your head around him being
possibly utilitarian you cannot even approach it and understand it by thinking secretly that he's still really just a "selfish evil bastard" (even when it is proven to be true later on).
Therefore, from a maintained utilitarian perspective of Mengsk, what you've mentioned above would not happen unless Mengsk misjudged peoples motivation and the overall situation regarding Terran affairs (which is entirely possible as well but that is discussing something else entirely again!).