Wow it's been a while since I've seen StarCraft II. Zagara is definitely more of a threat but that doesn't mean she isn't an ally. The way the Zerg operate they follow strength up until they can surpass that strength. That way the Zerg are always led by the most powerful leaders who can secure essence for them. In this system there's no such thing as our human concept of loyalty but at the same time the Zerg are pure. The only thing they care about is essence so all you have to worry about is getting a constant supply and to always be the one with the most supply to have the most power.

One day Zagara will be replaced as well and it also won't be considered a betrayal by whoever it is that kills her.