'Because a Siphon is never taught to feel guilty about eating a mermaid.'
The first thing you are taught as a Siphon is to never feel guilty about the consumption of the beautiful and yet naive mer-people.
The second is that you will never drown, but never walk into the ocean.
The third, your prey will always wear a necklace of blue and green around their neck.
Finn knew these rules like the back of his hand, right up until he stumbled upon the young, charming and sweet merman Finn. And suddenly, those rules began to look different, like chains enslaving him to his own nature. A Summer romance can give a lot of things. Like love. And freedom.