This card won't do anything if the unit you target is already corrupted. Because it says "Then…", the first effect has to occur before the second. In this case, that unit has to be corrupted first, then it will deal damage equal to its power.
The second part of your question is a little more difficult. I would say yes, your unit will still die. The reason I say that is because the effect of dealing damage equal to the power will exist regardless of what happens to the source of the effect once it has met its trigger conditions and the action taken.
I have been wrong before, woefully, so hopefully someone else chimes in.