Case in point you don't really support the legion if you do this. You only support yourself. This is valid reason to join the legion and the DB but you sure as hell arn't supporting the imperials as the topic creator asked.
Not necessarily. I support the Empire and believe that a united front under its banners has a better chance of taking on the Dominion than a rickety alliance between countries, but I feel Mede hasn't done a good job leading the Empire and has lost his Divine Right (if ever he had one) to lead. It's more detrimental to the Empire to have him on the throne and I'd be doing a disloyalty to the Empire to let him continue his ineffectual rule. Meanwhile, Motierre and I have some bad blood, and there's no way I'd let him on the throne, so I need to take him out too.
It comes down to a matter of do you believe that the Empire you took an oath to serve is static, and locked into whatever the status quo is at the time, or an ideal that one has to strive for. While my character is largely acting out os self-interest and a desire for power, it isn't so she can rule as a tyrannical despot. At this point in time, the Empire is unable or unwilling to keep the Dominion out of its borders and defend its people. I have no way of knowing if, when, or who plans on changing this, and so I'm forced to believe that no one is and it's never going to happen. On the other hand, I know I have the power to make a difference and to challenge the Thalmor, but I can't as long as the ruling class forbids it, which makes me think it's time to change the ruling body.
From my point of view, the Empire isn't supporting the Imperials, Nords, Bretons, or anyone else loyal to it to the extent it should, but by removing the ineffectual rulers and defending my people, I am supporting the Empire and its people.