[quote]First off they needed an idea of how to defeat the flood. The didact suggested that they build a protective barrier around every planet.[/quote]Not around every planet. The Didact's plan was to create artificial planets called Shield Worlds in locations of strategic importance to the Ecumene. When under siege, a Shield World could be used as a place of refuge for nearby planets and star systems. At the same time, they served as a base of operations to conduct military operations in the surrounding area. A promising plan indeed. [quote]Instead the forerunners kill all life in the galaxy.[/quote]The eventual choice by the Ecumene Council to use Halo instead of the Shield Worlds was one of politics and corruption, not of logic and merit. During the course of the human-Forerunner wars, the Builder rate became very wealthy and powerful. By the end of the war they had a heavy sway over the Ecumene Council. Using this sway, the Builders lobbied for a solution to the Flood threat that would benefit their rate the most. This happened to be Halo. The construction of Halo essentially cemented the Builders as the primary power in the Ecumene. [quote]Also before they used the rings the took the eggs of every life form in the galaxy so they could repopulate. Now that's a good idea....[/quote]Not just eggs. They took DNA samples, conducted genetic sequencing, and took live samples (this is the primary part of this plan). [quote]except for the fact that some idiot decided to save flood eggs so they could "study" them or some crap like that.[/quote]This decision was not taken lightly, and the samples of the Flood the Forerunners had were almost destroyed. The main reason they were kept around for study was because the Flood did not originate from our galaxy. If the samples were destroyed, and the Flood threat was wiped out by Halo, sure the galaxy is safe, but then the next inhabitants have no way of studying or determining weaknesses to the Flood if they were to return. The Flood did after all arrive in our galaxy from beyond its borders. For all we know, our galaxy could be an island in a sea of Flood infested galaxies.

Alright, so let me address your two points. 1. The Didact was mainly shut down because of the way the forerunner hierarchy is structured. Also, his plan was more idealistic (it may or may not have repelled the Flood, since they're so infectious). The Master Builder, on the other hand, had a more practical (I will explain) approach and was the most powerful forerunner on the social ladder. It would be one thing if a warrior servant like the Didact went up against a normal Builder, but the Master Builder was basically like the president- he had more influence than most, if not all, others. As to the practicality, the Flood really couldn't be stopped. The only reason humanity was not defeated was that the Flood wanted to give humanity a shot at attaining the Mantle, which is explained at the very end of Primordium. So, in light of that realization, Faber decided that he'd strike a different kind of blow by eliminating the Flood's food source. 2. The forerunners actually weren't out of the ordinary when they preserved a few specimen. Since they never anticipated that anyone would be so naive as to release the Flood (ie the covenant), they kept a few specimens for studying. Plus, we do the same thing with viruses and infectious bacteria- we trap and study them to find a cure.

As Cheese pointed out, they had their reasons; however, the story of the Forerunners is one of hubris and power.