And there´s the slangy "lullabye" meaning to commit murder someone, or hurt someone severely (if you catch my drift), in a way that it could kill the one being subjected to such act. This is found in some lyrics, for example. This would be similar to the slangy counterpart in Brazil, in some parts of course, where people say "cantar pra subir".
That comes certainly by analogy, from the fact that lullaby means "a soothing song with which to lull a child to sleep or quiet them".
But then, yes, lullabye seems like having its way with the internet people, meaning lullaby as well.
By the way, speaking of grammar points, "lullabies" is the plural of "lullaby". Perhaps the sound of both "lullabies/lullabyes" has helped to make the second a more widely used form. Again, by analogy (now from the sound of it) but then, it´s just a theory of mine.
And, of course, its use in this way is not standard English, but used sometimes regardless of the grammar rules.