I thought differently. If he removes his own mask, it might be for bragging rights so he can prove it was him who carried out this attack and not someone else. He'd want to brag that he planned, stalked, carried out successfully and escaped without being caught for years to come. He'd be bragging and likely if he uncovered his face, he'd be bragging to people who knew him. He wouldn't care if people who didn't know him / could not recognize saw his face. He'd want to show the people he did know that he had done this and gotten away with it. Why? It would bring "street cred" amongst his criminal peers.
Also, its possible that he did that because perhaps the victims could have identified him had they seen his face. That would add to his "bragging rights". It would be another way to lord his power over them - to laugh at them among others who may have known the victims in some small way.
It doesn't make sense he'd want to take the mask off for his own viewing later on. He'd know he was there, and had done this. He'd be reliving it for years in his mind. If he took off the mask, he did it for bragging rights in my opinion.
ETA: writing this and reading it back made me think perhaps there was a bet. Someone may have bet him he couldn't carry out an attack of this magnitude, and so he did. And he removed his mask at the end to prove it? Just a thought.
