This is pretty good for a light read. Without getting all deep with psycobabble like the guy below me, this manga has good flow and the characters move along with that flow, the art was good and the ending was ok.

Read if you like a good quick story that can keep you on the edge of your seat.

It is not that humans can be manipulated easily. Our physche is much more complex than that. The desire for risk, violence is to get adrenalin. Some become addicted to the adrenalin rush. It comes from one of our most basic instincts. We were hunters and gatherers at one time and were also hunted. Adrenalin surges may have been a daily occurence at that time. Now, being at the top of the food chain, there is no such regular adrenalin surge. As humans, we desire some excitement to get the adrenalin flowing. Some need more than others and like any other stimulant, it can be addictive. Also, like any other chemical in the body, we can become used to a certain level and thus require more. Thus, the need for more intense "excitement".
This is just a basic explanation. Please study some phychology and human physiology and human sociology for a more indepth and accurate description.
I hope this explains why humans appear to be easily manipulated as one person stated. Of course, this is not the only explanation, but I feel it does pertain to this story. This story has a lot of other psychological issues as well. They all add together to make an interesting story. Since it is so relatable, it is also a somewhat scary story. Great story. Enjoy!