Blunder in Speed Chess

In a game of speed chess against Vshape05 of Italy, a blunder allowed me to mate in nine.  My opponent, who played black, failed to find the best move.  Here are the moves of the game along with my commentary:

1.  e4 e5
2.  Nf3 Nc6
3.  Bb5 a6
4.  Bxc6 bxc6

I usually retreat my bishop to a4 but in this move I decide to capture.  Black does not make the correct capture.  It is best to play dxc6 to open squares for the dark-squared bishop and queen.

5.  0-0 d6

It is possible to play Nxe5 but I decide to castle instead.

6.  d4 Qf6

With the black king in the centre, I want to open up the game.  Black's move is a surprise.  I expect exd.  The square f6 is a common square for the king knight.

7.  dxe dxe

Now I control the d-file.

8.  Bg5 Qg6

Black blunders.  It is necessary to counter my control of the d-file with Qd6.

9.  Qd8#

With the support of my dark-squared bishop, my square delivers mate on the back rank.  Black fails to prevent the control of the d-file.  This leads to mate in nine moves.

Black makes three mistakes in this game.  One is the capture bxc6 which is weaker than dxc6.  Another is the sixth move, Qf6 which deprives the knight of an excellent square.  The worst mistake, however, is undoubtedly Qg6 which allows me to deliver mate on my ninth move.