Marcus, fell in love with Didyme. Aro's dearest sister. This was not unusual; given the way she made people feel, lots of people fell in love with her. The difference was that this time, Didyme fell in love herself.

The two of them were tremendously happy. So happy, that after a while, they no longer cared about Aro's plans for domination.

After a few centuries of living with the Volturi, Didyme and Marcus discussed going their own way. Starting a new life.

Of course, Aro was well aware of their intentions. He was not happy about it, but he pretended to give his blessing.

Then he waited for an opportunity to act, and when he knew he would never be found out, he murdered his sister.

After all, Marcus's gift was much more useful to him than hers had been.

This is not to say that Aro did not truly love his sister; it's just that a key part of his personality is the ability to destroy even what he loves in order to further his ambitions.

Marcus never found out that Aro was responsible for Didyme's death.

That is until a specially gifted human stumbles into Volterra, Italy in search of the depressed King.. ~