Thanks so much for your curiosity! I'll try to answer those questions for you.
Akano doesn't really have a deep reason for killing the people he does, he just heavily enjoys the colour of people's blood. If I had to give him a specific reason, however, I'd say he does it because he feels lonely. He has no real friends and therefore forcefully takes people to give him company, however he knows they aren't truly his friends; making him kill them.
Alva is another girl he kidnapped, but he didn't realise that she was also a demon. He kept her for a month before she tried to leave, causing him to feel betrayed and stab her. Alva, being a demon, didn't die from this and decided to stay with him to assist him with his killings.
That's a short description of it all. If I ever continue Akano or Alva's story in some form, I'll be sure to develop on this backstory.