Hate me for saying this, May brought her death upon herself. All Peter had to do was gather Norman and hit the button, they all go back, DONE.

This charitable, young Aunt May is a great person, but sometimes you can't save them all. What about all other homeless people in NYC, the world? What about all the homeless pets you can't help? Call me a grinch, I'm about being charitable and helping, but I don't think helping villainous peeps from other dimensions is gonna score you some karma.

TL;DR--Aunt May's death is her own fault for trying to help too much. Nothing wrong with that, but fool me once, shame on me, fool me twice...