Mengsk was never shown to be like this. Instead the opposite. Like I said, he has been shown to take into his fold anyone he can use, and tend to keep them. He keeps duke even if Raynor and him agreed he was not to be trusted.
I assume this idea about mengsk comes from sc2. Maybe he does turn to be like this, but at that time he wasn't shown as having that character.
Watch this
https://youtu.be/-j3X3EuCf98?t=28m0s 28 minutes to 33:33.
1. It seems the protoss and Zerg had a huge battle over there. Mengsk didn't want to take any part in that, or maybe he doesn't want to be implicated. Maybe he wants to believe the Confederate were helpless and weak, and that he can do better. So he left with his entire fleet before they get swallowed up in the chaos.
2. Several dozen protoss warships vs billions of Zerg. Imagine tarsonis as a wild battleground, and only a strike force can penetrate.
I'd imagine there was only a small window for Kerrigan to do her thing, and it was spent.
I'm not saying Mengsk wouldn't do this kind of thing, and abandon soldiers when he has to, or pulling them out would get in his way. But what I have a problem with is the idea that he intended to kill her by sending her down there.