Try playing through Halo: Combat Evolved on Legendary difficulty. Completing the game on normal doesn't do the game justice, since it's quite possible to simply run in and blast everything to pieces with your Assault Rifle. Legendary is a completely different experience, and brings out the true charm of Halo. Enemy AI is boosted to the point where your only way forward is to think patiently and tactically, and each scenario tests your skills to the limit. It is by no means the best game ever, but is still an exceptionally well-crafted FPS, that is still a joy to play four years on.

As for your plot point, the Humans and Covenant that arrived on Halo both brought ample means for The Flood to escape (the Cruiser that you infiltrate in the level Keyes being a good example). If the parasite had managed to escape on one of these vessels then it would have been difficult to track them down, and they would have been able to spread throughout the galaxy much like a virus spreads through the human body. The Halo weapon acts as a last resort, when all other means of containing The Flood have failed. Rather than leave the galaxy to an enternity of Flood infection, the ring eliminates all life with sufficient biomass to sustain the parasite, therefore starving it. This sacrifice halts the spread, and the galaxy slowly rebuilds itself through the natural process of evolution, meaning that sentient life will one day form again.

The important thing to stress is that Halo is a weapon of last resort, and not the only means of containment. Without this little detail the game's plot turns from 'pretty poor' to 'absolutely crazy'. As it is, 'pretty poor' is a nice way of summarising the plot device explained above.

But the game as a whole is an excellent example of how to make an FPS. The game doesn't have any outlandish gimmicks, but is simplt a well-made title in which basic things are done very very well. Give the game a chance on Legendary, and you might see what I mean.