There is absolutely no shred of doubt that Iraq was a better place under Saddam Hussein. Since his demise, the place has been hell on earth. Scores of innocent people being killed in bombings and shootings is still a weekly occurrence and has been for the last 10 years. Iraq has turned from being one of the most educated and progressive Middle Eastern nations to being a perpetuating warzone with no real future in sight - but it's reasonable to assume that is the real reason Blair intervened, to profit from this scenario.
You have a good point but gassing the Kurds in Northern Iraq provided a different dimension. That was Saddam's downfall and it just needed any reason for the powers to act after that (i.e WMD or not) . Political opinions aside and whether you support him or not, it is (was) Saddam who should be tried not Blair.