Sounds like you fell through a glitch or something in the stage. Happens every once in a rare while, especially when you roll (did you roll?).
As for the kill, that's the way it happens when you jump off a cliff or something. If you just jump off unprovoked, its a suicide and doesn't count towards the other teams kills, like teamkills. But if someone damages you and you jump off, they'll get credit for the kill. Same if you like grenade yourself or something after someone damages you. I think a certain threshold of damage needs to be reached though.
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~Wigs~ WELCO METOT HENEX TLEVEL
I do what Ninten-Don't.