The "idea to kill" can come from any influence. Books, movies, games, friends, one's own thoughts etc... It is not the idea but rather the desire. Kids can be "provoked" by their parents when they perceive unfair discipline or that they are not understood by their parents. The human brain is not fully developed until about the age of 25. Up until this point human beings are not fully capable of logic or reason. Which is why kids often make terrible decisions and regret it when they are older. This is science. You may be interested in looking it up just for the curiosity. I encourage you to do so.
Anyhoo, kids who find themselves being relentlessly bullied by their peers (as I was) may become hostile and violent or they may simply withdraw and become very antisocial (as I did). The reasons why young people choose to kill others are many. Mental illness, depression, bullying, anger, betrayal, and revenge are among the very many things would drive a kid to kill classmates, parents, random people in a random place etc...
The idea that the "blame" should rest upon any one single shoulder is to our detriment. A broader look at all factors and making the best choices for our kids and communicating with them is the first step for parents. Schools should reevaluate how they handle cases of serious bullying, the media should take note of how their glamorizing shootings affect everyone who is watching. I could go on and on. But I will leave it there.