If you've read the author's other works (Suicide Island and Holyland), you'll see that Ren's thoughts about violence and death are pretty common. Fundamentally, humans value life. Even when murder or brutality is justified, doing so leaves a heavy mental toll. Even if you act quickly in the moment due to instinct/reflexes, it's not easy to feel okay with violence and murder and it can haunt you for a long time. That's just a fundamental part of human psychology. When you stop feeling that weight, you start to value life less and that's rarely a good thing.
This also seems to be something that the author went through personally and he includes it in many of his works. This manga isn't supposed to be purely an action manga, but also a psychological one. If you want a manga that focuses more on action, I highly recommend Holyland by this author. There's a lot of technical, bad ass fights in that one
Historical Sidenote: This mental recoil after seeing death was suppressed for most of human history, because death was so commonplace. Most infants did not survive till 6 years old, war was a way of life for many and starvation was always present. It's only after these problems were reduced in the modern age(~18th to 19th centuries), that we started to value life more.