I know that this is quite old, but it bears mentioning that consent is often a huge issue with magic, for more or less precisely this reason. We also know that it is important in the Potterverse as well, since Pettigrew's line was:

Flesh of the servant, willingly sacrificed, you will revive your master.

Not to mention:

Blood of the enemy, forcibly taken, you will resurrect your foe.

This shows that consent is important on both ends. Imperius could easily not count as actually giving consent.