"Mama, I just killed a man."
"Mama, just killed a man."
I'll ignore the blasphemy on the GREATEST SONG AND BAND OF ALL TIME!!11
But why would it be "Mama, just killed a man" with a comma. That would only make sense if some guy named Just killed a guy. Either that site is wrong or Queen are idiots.
Basically, a "," in song lyrics indicate a pause, not necessarily a change in subject. Taking the rest of the songs context into account, it is as if the singer was saying "I killed a man" there regardless. Which explains why the lyrics are so often misquoted; the "I" isn't actually in the lyrics, it's just inferred...
"Just killed a man!"
The speaker is indicating that he just killed a man to his mother. "Just killed a man" has a gapped subject.
Here's the video, listen for the "Mama, (I?) just killed a man..." at 1:00.
Mama, just killed a man
Put a gun against his head
Pulled my trigger, now he's dead
Mama, life had just begun
But now I've gone and thrown it all away
Didn't mean to make you cry - Speaker made his mother cry; wouldn't make sense if she killed a man.
If I'm not back again this time tomorrow
Carry on, carry on, as if nothing really matters