Often story-telling uses the mask to make us see a character as a simplified ideal.

Evil, bad, the enemy, the opposition.

I agree with Fury that in this instance, Ben was using the mask to make himself that ideal. He's clearly conflicted, and struggles to accomplish this goal that he thinks needs to be achieved (for whatever reason.) He has doubts.

However, with the mask he is the dark side. He is his grand father. The mask represents the simplified ideals that he wants to embody instead of the weak, unsure character he is without the mask. It's almost two personalities.