I always say that this is the series that wasn't meant to be a series but I can take that one step further and say it became the violent series that wasn't planned. Originally We're All Animals was meant to be a standalone book but as the story moved forward, it became clear there was much more coming down the wire. With each book, the series grew until it became brutally violent.
The thing is that this isn't so unusual in our society. Many things become normalized over time because they are slowly built into everyday life. Things escalate. Smoking starts with an occasional cigarette often with no intentions of becoming a habit. On a more serious note, domestic abuse starts with an insult, a shove or some form of control. However, many things are insidious. They grow over time until it becomes the new normal. We regularly see it in our culture, especially in the news.
The interesting thing that I noticed while working on my 12th book is how Chase - the character who has been along for this journey since day one - started off so naive and shocked by the same violence he later comes to participate in. We see this in the real world too. It's not just fiction.