Why Mengsk wanted to defend the zerg against the protoss in Tarsonis again?

It's been like 10 years since I played the SCI storyline and SCII is out tomorrow. I watched the SCII trailer on youtube and parts of it I remember was the battle in Tarsonis in which mengsk wants to use the zergs to destroy the confederacy and crown himself emperor.

The part I don't remember is that if the protoss is going to kill all living things on Tarsonis anyways why does he still have to send Kerrigan to defend the zergs against the protoss to let the zerg kill the confederacy? He can just use the protoss to kill both the zergs and confederacy on Tarsonis since the protoss don't care they just want everything dead anyways.

This is confusing I don't understand, but this part is basically the classic scene of SC which led to the downfall of Kerrigan opening the brood war chapter so I don't want to miss.

Thanks

 

Because the protoss refused to kill all living things on Tarsonis, as was Mengsks original plan. Instead, they sent down a ground army to stop the zerg from killing everything themselves.
Kerrigan had to hold off the protoss, so the zerg could destroy the entire world.

 

But I thought the protoss kills all other species to avoid the zergs from assimilating them and get more advanced biologically? Isn't that what happened to Chau Sara at the start of SC?

 

It's what happened at Chau Sara and Mar Sara, but apparently Tassadar had begun questioning the policies of the conclave by that point and didn't want to kill more innocents

 

Ah I see. Thanks for the quick answer. I think Tassadar made a huge mistake. If he instead decided to go for purification and exterminate all life forms on Tarsonis then probably brood war wouldn't have happened.

 

"The road to Hell is paved with good intentions."

 

En Taro Tassader, Selendis.
Kill all humans, Jim.