[quote=thewizardguy]
They didn't. Kerrigan was converted because the Overmind wanted to use the pschic powers Kerrigan possessed as a weapon. It could adapt to the guns and blades, but it wanted to use Kerrigan to develop a way to defeat it's ancient enemy, the Protoss.
[/quote]

then why did she have to forma a psionic link with the zerg in the first HotS mission in order to control it?