Except the way they wrote Mengsk and the Zerg hierarchy, they were competent generals. The reasons for Mengsk doing what he did, aside from abandoning Kerrigan and the people of Tarsonis, was rather logical, and the Zerg Overmind and its Cerebrates were also competent enough as leaders, with you as a Zerg Cerebrate protecting a tier-one asset, engaging the Protoss and decimating them on Char, and sparking the Fall of Aiur, with the other Cerebrates and the Overmind not being as brain-dead as Tassadar was.
Considering how Tassadar is a weepy baby with a martyr complex, I'm not surprised he chose to go out that way. Going full AN HERO instead of just using an assault to mask a Protoss Arbiter teleporting Zeratul in is pretty much something I can see him doing.
From my PoV, SC1 is fine, it had some good themes, and Brood War is cool, but it has a few problems in the Zerg and Protoss campaigns.
Hence why I think the UED should have been involved in SC2 as the primary good guy faction for Wings of Liberty. Stukov was already de-infested, why not have him come back in SC2 as the big good for the humans instead of having the most noble human leader be a drunk hick in a stolen cruiser?
I think, for a Zerg antagonist, they should have brought the SC1/Brood War Cerebrate player character (Araq) back in a Hybrid body. Like say, Duran managed to save him before Kerrigan decided to ice him, and placed him inside a Hybrid Reaver's body:
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Sometime during Heart of the Swarm or Legacy of the Void, he takes control of some of the Zerg forces, and he continuously throws Kerrigan's evil deeds in Brood War back at her face, especially when she tries to act all reformed.
Kerrigan: "No......I'm not that Queen of Blades anymore!"
Araq: "Sure you are. You're the same bitch who betrayed her friends and bungled every military advantage we gained. Then you had me clean up after your hijinks with the Protoss and the Dominion while we were fighting the UED."
When he sees Kerrigan fighting alongside Jim Raynor, Araq throws Fenix's death in their faces, and Araq asks Jim about his promise to kill Kerrigan. Araq talks about how Kerrigan ordered him to kill Fenix and make life hell for the Protoss forces during Brood War. Then when Artanis fights him, he laughs in the Hierarch's face and talks about how weak the Protoss were when he invaded Aiur, decimated entire cities, and razed the Xel'Naga Temple to the ground.
Artanis: "You will do no better than any of those other Cerebrates that fell before our might!"
Araq: "Don't make me laugh, boy. I burned down the most sacred site in your homeworld so that the Overmind could nest over it. I eradicated entire legions of Protoss warriors on Aiur as if they were rabid dogs being slaughtered for pig feed, just as I decimated the armies of that pitiful fool Tassadar on Char. Your people at their height could not stop me. How could a has-been like you do better than your beloved hero Tassadar?"
*Cue Artanis exploding in nuclear rage against Araq while Zeratul desperately tries to hold him back*