To fight Satan.
To rescue the souls of the faithful from the power of the enemy, especially at the hour of death.
To be the champion of God's people, the Jews in the Old Law, the Christians in the New Testament; (therefore he was the patron of the Church).
To call away from earth and bring men's souls to judgment.
If this is Islamic terrorism, the message the Muslims communicate is quite clear: St.Michael can't protect the infidels.
However, if this is a lone domestic nut, it may also be a clue to his religious upbringing (more likely Catholic or Orthodox than Protestant).
I find it interesting that there was a cemetery across from the first Michael's. The Gate of Heaven.