So I watched all the cut scenes from Halo 1 again and read up on the story of Halo 2 since I never played that one and there's just something I don't understand: why even bother building the Halo's?

If I'm understanding this right and I admit I could be completely off since I didn't bother playing Halo 2, didn't care too much about Halo 1, and haven't touched Halo 3 yet, but the Flood needs sentient life to survive.  They keep eating and reproducing as long as they have life to take.  In order to combat this, the Forerunners created the Halo's to stop them.  To stop the Flood they made the Halo's kill everything across several galaxies so the Flood just starve.

So my question is why didn't they, the forerunners, just move?  If I had the choice of simply move or build these elaborate devices that kills everything in the galaxy I would just move.  By not being there the Flood can't eat.  When the Flood can't eat, they die.  I guess that isn't as cool as making a device that kills all life in several galaxies and not nearly as cool as making seven of them, but why wasn't just going somewhere else an option?

the flood can chase them

in halo 3 the flood use the covenant transportation to come to earth, flood are able to somewhat use the mind of the guys they feed on, well they can use their technology in all, so the flood can chase them and would just engulf every possible living thing for who knows how long

Maybe the flood had the ability to follow? Maybe they were spreading and needed to be stopped? And last, but not least, when was the last time you saw anyone walk away from their property because of some intruder? Would you abandon your home to cockroaches or would you spend great time and effort to kill them? I'll admit, however, that they story could have been woven more tightly across all three Halos. I don't think Bungie or Microsoft expected the kind of response they got and had to iron out some of the inconsistencies along the way to Halo 2 and 3.

Why not just make a giant EMP then and say screw you guys then? I figured the Flood had some way to move around but I would imagine that if you have the power to kill all sentient life in the galaxy, you can figure out a way to disable engines.

Lol, hmm maybe the covenant ships immune to emp some how, that would make no sense O.O

that or flood could rebuild new engines or something i dont know =P i just know the flood would follow them

If the cockroach problem presented me with the options of die or GTFO then yes, I would gladly just abandon my property.

I don't know, just the way they make the Flood seem it just seems like it could live real long without life to feed on. If it could live a long time, then it could just migrate to a galaxy that didn't get touched by the Halo's (like Earth) and continue to spread. I assume since they can follow they can go as fast as the things their following and since they can travel between places pretty quickly in the game the flood can move that quickly then.

Pretty much what everyone else is saying. The Flood had access to the same technology as the Forerunners, and running wouldn't have worked.

It's all a bit hazy, but I'm sure Halo 3 explains this at least a little bit. I've been playing with the idea that a bunch of the Forerunners places themselves in stasis and woke up waaaay later becoming the humans on Earth, but I HAVEN'T PLAYED THE GAME SO PLEASE DON'T TELL ME IF I'M RIGHT OR NOT.

This metaphor becomes accurate if the cockroaches can run as fast as you can, have bigass SUVs and WANT TO EAT YOUR FACE. Also they'll take whatever pesticide you have an use it right back at ya, and don't even think about getting a gun or you'll be in for a world of hurt.

The problem I have with the Flood can follow idea is that why didn't they after the Halo's were used? Earth was saved and the Forerunners knew where Earth was. Why didn't the Flood go to Earth?

The only reason I can think of for that is that the Flood have a short lifespan if they aren't feeding. Since they have the short lifespan when not feeding, they can't travel as far as Earth (even though they could already travel fairly fast where Earth is almost a skip away). If that's the case though, why didn't the Forerunners just pull a Battlestar Galactica and keep moving for a few months? It's not like they didn't have the time to build and plan ships that could transport their civilization since they had the time to research and build seven Halo's.

 Oh I'm not arguing the flood is scary as fuck, I'm just saying that if I was presented with the option of killing myself and everything else or just leave everything behind and move then I would be perfectly happy with moving.