What makes someone want to murder?

There are so many reasons why someone murders another person. It could have to do with drugs, an accident, mental illness, and so many others. One though is health issues such as tumors or malformation of something in the pain. Some people such as Whitman in the Time article went on a mass kill. Killing 17 people and then his family. What would make someone do such a horrid crime? Whitman had wrote a suicide note after his killings and the killing of himself, saying for sometime now he hasn't been able to stop these thoughts, he doesn't feel like himself and he wants to donate his money to do research on mental illnesses. Whitman clearly stated that he didn't feel right and apologized for what he has done in his letter. If he didn't think what he was doing was the slightest bit wrong I don't think he would have apologized. When  they did Whitman's autopsy they found a tumor and a malformation of a valve pressing against his amygdala. The amygdala is what controls emotion in the brain. They believe that the tumor pressing up against the amygdala could be what contributes to Whitman's actions. How can one control themselves if what they are feeling and expressing is really what they are felling and expressing? Many murders and their actions have been linked to malformations, tumors, traumas, and brain damage from previous injuries. Which leads to was the crime a true act of evil or would it have ever happened if their illness had been treated. Its a question that is still hard to have a complete answer to due to others actually committing the crime knowing why and having done it for a specific reason.