I think it has something to do with clan affairs. Makio has been doing lot of shady things in the past & the present- she's snake I think (can't remember correctly was it her form). Makio's dad Kunimitsu is the leader of the Japan's heavyweights.
Mermaids are on the top when it comes to madararui species- top heavyweightS.
Vulnera is well only half mermaid (marmaid+lion), but still nevertheless topnotch.
So politics and stuff.
I think.
Have to get rid of someone who might take over to be leader of the madararui
Also Kunimitsu and Makio wants Kunimasa to be the successor
Ohhh, yeah that would make sense. Damn, that Shinobu has really bad daddy issues, to do that to someone he's in love with. Thank you !
Shinjuu is the answer.
I'm liking Shino and V.B's story, but I don't really understand why was Shino trying to kill v.b when his job was to teach him how to get a human form...