Wrap your head around it...
I understand that people don't understand the need of the Flood if the halo is capable of destroying sentient life.
How about this, Halo destroys sentient life by RELEASING the Flood into the galaxy. After all, the Flood feed on sentient life. Once all sentient life has been wiped out by the Flood, what do the Flood do? Die of starvation. Leaving planets free for colonisation. The perfect weapon. And we ARE told that the way to kill the Flood is to activate the halo.... well in a round about way this IS achieved. Destroy the Flood by starvation by activating the halo and killing sentient life.
That works (in my opinion!)