Mengsk being a power-hungry dictator is a given, but the driving force behind the story, past the first Terran campaign, was never presented as a liberation narrative eg "Mengsk is a tyrant! He does all this crap! He needs to be overthrown!"
Mostly because Raynor has better things to worry about. First he tries to see what is it about the dreams he receives about Kerrigan, who was a friend to him. His force gets decimated and he meets Tassadar, and becomes a liberator for the Protoss. The Zerg are the big threat in the game, not Mengsk. The first story was an introduction, and later the Dominion becomes a minor villain who interferes with both the Zerg and Protoss campaigns, but they've had their moment, everything now is about the cataclysm between two ancient races. It fits, given that the Terran campaign was mostly an introduction of all the characters who will play the main roles in the whole Zerg/Protoss war.
The driving force behind the story was Kerrigan enjoying being a Zerg and deciding to go on a killing spree against her former teammates. Because "vengeance."
What?
She goes to the Amerigo to unlock her dormant psionic powers. Then she gets some leeway for herself under the supervision of a few Cerebrates so that she can grow and gather experience. Her slaughter of the Protoss was relatively untimely (what with the sneak attack that ensued), but still within the Overmind's plans (who wouldn't want the Protoss off the planet?), and the whole stupid "let's kill a Cerebrate for no reason" plot gets the Protoss homeworld of Aiur exposed and the Protoss back broken. Then she's left behind on Char to clean up the backline because she's a pretty damn minor character. She doesn't even appear in Episode III. When Raynor shows up, he's all about finding new allies and his final monologue is about the Zerg wiping out his "home, family and friends"; he doesn't explicitly name Kerrigan. Kerrigan fades as a presence way back in Chapter 2, and becomes a potential sequel hook (in the form of Brood War) only in the ending blurb. She was infested to the core, mixing up human cruelty with the Zerg's conqueror nature. She was an agent of the Swarm, a potent general, a strong evolutionary link for the Zerg (and Zerg are all about evolution). Kerrigan's vengeance plot only becomes a thing in BW when Terrans kinda become a dictating force in the universe (standard SC is a grand Zerg/Protoss battle with Terrans as a small tipping point in comparison; the perspective in Episode II and III tells us that neither Zerg or Protoss particularly care for Terran politics, and Raynor's Raiders are firmly established as second fiddle, support to the Protoss).
It is this leitmotif of the story that is the problem with the entire Starcraft narrative, and it started with Starcraft 1. What's wrong with Kerrigan holding a grudge against Mengsk? Nothing - had she remained a simple villain and plot device for Raynor's development. But when it becomes the main motivation for the narrative, then everything is wrong with it because it is not sympathetic, and therefore not emotionally compelling. It situates the entire narrative of Starcraft on a stupid case of "hell hath no fury as a woman scorned." Plus it removes all that was sympathetic about Kerrigan's "death" to begin with and makes Raynor's love for her hard to believe.
Kerrigan wants to unite all the Swarm and relish in the power. Not a bad goal, as far as video game villains go. To that end, she isn't above manipulating everyone to meet their ends. Dealing with Mengsk was a big part of that plan, but it was resolved and didn't need to be carried on over to SC2. Kerrigan, by the end of BW, could have easily just continued the usual Zerg legacy and go on with furthering her power, fully in line with the whole "the Swarm evolves" schtick that she seems to like quite a lot (a lot about Kerrigan's storyline in SC1 and BW is about her gaining more and more power and control, starting from the whole Amerigo bust).
What they didn't think was how shallow, ultimately, Kerrigan's character and motivation actually were, such that building a story around it ends you up with Mengsk being the most sympathetic character.
This is only if you honestly believe that Kerrigan hadn't already had delivered enough comeuppance upon Mengsk, first by making him out into a total fool in True Colors, and again, in Omega, where he gets spanked again with nothing to show for it other than a lot of expended "favors and concessions", and that Jim and Sarah were ever actually romantically involved and it wasn't just Jim Raynor starting off as a horny young fool and then becoming a hard-ass with the biggest grasp on humanity in the series (which shows in how he's the link between Terrans and the Protoss, and how later). SC and BW make me a Raynor/Fenix shipper, not Raynor/Kerrigan. Raynor worked with Kerrigan because he thought of her as the lesser evil (because he already knew, or at least thought he knew, the extent of her evil), and because he thought their goals were in line (since he bought the tale about the UED being literally worse than Hitler).
Way, way more could have been done. Mengsk somehow manages to become the ruler of an unified Terran Dominion despite being a massive dick with balls and, by the events of Brood War, reduced to a crippled, desert planet of Korhal, his empire in shambles, with massive conglomerates like Kel-Morian Combine somehow not deciding to capitalize and use the grace period Kerrigan left them to establish a new order in the Terran sphere of influence. It could have been a story about both Terrans and Protoss collecting their broken up societies together, as I assume the tensions regarding Dark Templar and the matter of Aiur could have been resolved in a much better way. You could even have Raynor work with Valerian Mengsk and somehow work off their uneasy relationship and coming to terms with one another, with Valerian succeeding "Emperor" Mengsk and learning from the mistakes of overzealous ambition. And once again, they'd bring the Zerg to a stalemate, or even Raynor would finally get his climactic kill on Kerrigan. And then the Hybrids could come into play somehow, being a massive wildcard; to throw the wrench into everything.
And again, Starcraft's story wasn't that good or that deep; it was very entertaining because the presentation was amazing and fit the genre. It was pretty good for a video game, and the story of SC2 could have been a big LoGH/Gundam/Tekkaman Blade clusterfuck for all I care; anything but all of this bullshit.
To put it simply:
The problem is that we're supposed to believe that Mengsk abandoning Kerrigan was such an atrocity - compared to all the other shit that went on in the game - that it becomes the central motivation for all the main characters over the course of the next six campaigns, except for the Protoss who don't give a shit.
This is simply wrong. UED wanted Mengsk removed to become *the* Terran authority in the sector. Kerrigan wanted vengeance on Mengsk, enacted it and went on to a much grander plan, which was to become the ultimate ruler of all Zerg swarms. Raynor and Fenix struck an alliance with Mengsk and Kerrigan only because Kerrigan convinced them that UED is the greatest evil in the known world.
The deal with Mengsk in the campaigns is that he is always put into irrelevancy. He ends Episode I triumphant, and then his presence is a token annoyance as a minor villain, represented by General Duke putzing around somewhere here and there, with the Zerg and Protoss cosmic conflict being the actual big deal. Then in Episode IV, he doesn't show up again - the Protoss are too busy with their shit, and the token Protoss vs Terran mission is used to foreshadow the UED. In Episode V, the UED topples Mengsk, but the grand quest is about reclaiming the Overmind and the ending cinematic doesn't mention him. And in Episode VI, he's a puppet to Kerrigan, Kerrigan gets his revenge on him and leaves him to rot, and then he is unheard from again until the final showdown, where he's leading a battered fleet of ragtag soldiers. And again, when you beat Mengsk and Artanis, they just go with "I'll get you next time,
Gadget Cerebrate, next time!", but the real shiver-sender of the story is the way Kerrigan deals with DuGalle.
If the whole story was about Mengsk from the beginning, we wouldn't be having the "Dearest Helena" cutscene and Kerrigan wouldn't sadistically inform DuGalle that his entire fleet will be wiped out and there's nothing he can do about it other than struggle as hard as he can.
Mengsk is fucking irrelevant other than a tool for squabbling around some Terran politics and Terran loose ends. In every storyline, he is quickly disposed of.