I was playing as black. Why is Nxa2 a blunder in this position?

And which would be the right move for black? Try to guess.

nxa7 bd5 kh8 qh5 bh6 qxh6#.
right move is probably rf7 to prevent any mates

More or less. White answered with Rxc7, but I guess Bd5 is also an appropriate move. And yes, best move for me instead of Nxa2 would be Rf7.