the "us" isn't just God but also His angels. He says "behold!" - who is He talking to? Himself?
what evil have the angels seen, and what good? Adam had now done the same thing, and specifically Adam had proven where his faith is - just like the angels who did not fall, had.
the view that Satan was telling the truth, that committing sin makes you godlike, is (imo) foolishly simple, frankly blasphemous and doesn't fit the text at all, because Eve is not included in God's statement, but only Adam. It's a response to what Adam had just done.