Non-spoilers, but based on speculation after reading the first two chapters:
Obviously the emperor is behind the attack, because he wants Chester to be fully devoted/loyal to him and only him by killing off his real family and the girl her likes.
Since it's also obvious he killed his own father and brother too.
Whether he has the power to do this alone or with someone else's aid is the only mystery, and how Karl will escape his grasp to be with his beloved dragon girlfriend.
I don't hate this, but it's too... stereotypical.
The emperor is obviously playing nice/the friend, instead of something more compelling to keep us reading and later it being revealed it was him all along because he was scared of losing his friend blah blah blah grey area.