What should happen next in the story I'm writing?

ok I'm writing this story, its like a demon human thing an I'm stuck can any one help??
here's what's happening:

'Marcus, you can`t kill me, I`m your sister!' Emma pleaded.
'Not anymore...............' Lifting Emma off the ground, Marcus took one of Emma`s knives and
stabbed her hand into the wall. People walking past, saw what had happened and tried to get
Emma down from the wall, but Marcus pushed them away!!
Marcus held Emma against the wall but soon they both disappeared leaving a circle of black
ash on the ground!
'What, where`d they go?' asked Alex.
Looking around, Eric saw all the people look at him, he slowly looked at Alex and hugged her
only for the same thing to happen to them as what happened to Emma and Marcus.
Serax stared at everyone, laughed and then a black bolt of lighting smashed him into the
ground, him vanishing within it!

I really need help what should happen next?

in my opinion, I wouldn't play around with Demonic stories....they tend to leave the wrong 'fingerprint' on your life.