[quote=Tyki]
then why did she have to forma a psionic link with the zerg in the first HotS mission in order to control it?
[/quote]

She didn't. She had to become the alpha, which required her to defeat the other swarm. The zerg are a hive mind, which will join any 'queen' that can control them, Kerrigan was bullyng them into becoming a part of her army.