After being restored to her human form, Kerrigan is contained and experimented on by Valerian. During this period, Valerian wants to find out whether she can still command Zerg, and asks her to take control of a drone and build a hive and some other units.
At the beginning of this she asks
“Are you really asking me to take control of a Zerg mind? Do you know what could happen?”
She makes the outcome seem unforeseeable, but we see that she has good control over them. So at this point this clearly turns out to not have been an issue (not sure if she knew beforehand that it would not be an issue). Further, however, she proceeds to make the Zerg attack their level of the facility, causing significant damage. Why?
She makes it sound like she wanted to demonstrate how unpredictable the Zerg are:
“Funny thing about the Zerg, Valerian, they never do what you expect.”
But they are not unpredictable at all, they did exactly as she commanded and stood down when she wanted them to. The main thing this seems to demonstrate is that the Zerg are powerful, which everybody was up to speed on. It also demonstrates that she can control them (Valerian's initial question), but that was already clear by then.