We could've been somebody
Thought you'd be different 'bout it
Now I know you not it
Can be taken in various perspectives:
The unnamed female betrayed ‘Ye by talking to the media about their relationship, showing him that she isn’t different from any of the other “bitches” that he had gotten with before. 'Ye is pretty heartbroken, as he was hopeful about their future before she fucked it all up. (Kanye West’s ex- fiancee actually talks a lot to the media about how she feels about Kanye’s current situation. And she didn’t mess it up; reportedly she broke it off with him after she saw naked pictures of Kim K on his phone.)
The girl who ‘Ye loved before his fame and success, didn’t wait for him and ended up having sex with other men and subsequently broke his heart when he came back to her. 'Ye expected better and explains that they could have been an item together had she not betrayed him.
Ultimately, since she has shown her true colors, he is going to treat her accordingly to what she has proved herself to be: a find, fuck and forget.
It is possible that the lyrics here are “Now I know you naughty,” and “you not it” also vaguely sounds like ‘united.’ This would link into the line, “We could’ve been somebody.”
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