Manichean views don't explain enough, although they do create engagement, which may be the primary goal.

A less angry explanation is that it is all of that at the same time. They want to help Ukraine's democracy, weaken a historical authoritarian enemy and feed their military–industrial complex. It's a balance of all of that in the interest of the people that elected them, like in any democracy. If something gets out of balance, yes they will probably retract their support before it hurts their country in some way, like any other country would. It's just Realpolitik.