The HL story as seen by Tecell:
Gordon freeman, employee of black mesa, much untapped potencial as seen by the Gman (agent of the administrator's organisation). Happens to be the unlucky sod who has to place the "sample Xen crystals" into the massive highly unstable generator/ test machine. Unlucky for him the equipptment and pre test theory fails and the sample causes a 'resonance cascade' (If u've ever played Star trek elite force on of the engineers in the engeneering section says "check the resonence cascade modulator" a little clip from/ tribute to HL
). lucky for him hes wearing this new HEV protective wear that helps him survive the accident. He fights his way learning what little he can of what is going on around him from what you play in HL. At the end of the game the Gman hires him because he alone managed to fight his way through black mesa and then single handedly defeat the last leader of the Xen (nihalith says at one point "I am the last you dont know what your doing" or somthing allong those lines). So the Gman hires him because of his achivements. The whole point of HL is that you see an epic story unfold through the eyes of one participant (in the case of HL the main participant, Freeman).
The whole point, as stated by 'Hazardous!', is that you dont know the whole story, you can only put together a few peices of the puzzle, which leaves the player to assume or conclude the rest making room for mods and whole mapping comunitys on which the story can grow and evolve. Like Blue shift and Opposing Force which stick to the primary story just from a diffrent perspective. Half life is an Enigma so to speak. HL2 (and 3) may answer a few more questions and probably generate a lot more, thus creating a never ending story which is just what Valve want. A story that never ends can never die and so the mapping and modding comunities will never cease to exist, or at least not for a very long time (hopefully lasting until appocalypse).
The whole story of Half-Life is based around that fact that you will never know the full story, only parts of it. so comming back to the original question, you will probably never know if the Gman wanted the accident to happen, or even if he played a part in creating the Resonance Cascade so that humanity could invade Xen. From what I saw in Xen in HL it looked like the Nihalith already planned some sort of millitary action because there were all those factory like places such as interlooper making Alien Grunts in pods, why build an army if you dont plan on using it? Bottom line is, all you know is what Freeman, Shepard and Calhoun saw, bits of the story never actually making a full picture, just parts of it. To truley know the full story you'd need to get inside the head of Laidlaw and the other members at Valve, somthing which will probably never happen. Maybe they didnt even make a whole story, perhaps it was just ment to be incomplete, a parcial view of what went on at some research base. Who knows...