So Jameel has this post up which argues that the story of Matisyahu screaming מי לה' אלי" after murdering an idol worshipper is pure fantasy. According to Jameel it was invented about 100 years ago by one of those maskilisha types your rebbe warned you about.
I can't say that I am surprised. Plenty of good things were brought into the world by maskilim [list on request] but what might shock is the idea that a myth might have been invented so recently. And in our age of mass communication, too.
(V hamayvin yaavin.)