My idea of TFA Megbird is that Nightbird was a cyber-ninja and used to be a student of Yoketron. She was considered to be one of Yoketron's favorite students and there were rumors that she was going to be his successor.
But that all changed when Nightbird and Megatron met and they fell in love. As the prize pupil of Yoketron, she was able to put up a good fight with Megatron, earning his respect for her strength as a warrior.
During their talks, Megatron had managed to get Nightbird to see how deeply flawed the Autobot system is. She even realized that some of her fellow Autobots are just as or even more cruel than the Decepticons themselves.
Nightbird even began to see that her beloved master was just as deeply flawed as the system they were apart of. But she still deeply cared for Yoketron since he basically raised her when she was younger and saw him as the only family she had.
Knowing her master would never accept her love, Nightbird eloped with Megatron, joining his Decepticon case and becoming his queen. Naturally, the Autobots saw this as an act of betrayal and labeled her as a traitor and a Decepticon lover.
The love story of Megatron and Nightbird is seen as a tale of selfishness and betrayal while the Decepticons see it as a story of love and and following one's spark.
