Is PTSD an excuse?

If you have not seen American Sniper, this post will spoil the ending. But if you have heard of Chris Kyle and know is story than you already know.
Chris Kyle is a United States Navy SEAL and was the most lethal sniper in the United States military history with 160 confirmed kills. While Kyle served 4 tours in Iraq he was awarded many awards for his heroism.
Kyle reached out to veterans who had Post-taumatic Stress Disorder and would try to help them. In 2013, Kyle and friend were shot and killed at a shooting range.
This week, the trial of Eddie Ray Routh opens. Eddie Ray Routh, who has PTSD, is accused of killing Chris Kyle.
Now the question everyone has is. "Will Post-traumatic Stress become the legal defense for ordering a man who helped other veterans battle the same affliction?"
PTSD is a very serious and dangerous disorder. But is this an excuse to why he killed Kyle and his friend? I don't believe that it is. I am very aware that it a serious condition but you don't just get an urge to pick up a gun and kill someone. If Routh wanted to shoot something, they were at a shooting range. Why did he decide to shoot people? In this article it states form Dr. Harry Croft, "The core symptoms of PTSD include irritability, agitation and anger. But the get angry at other stuff, usually, if there is any physical part of that anger, its with family memebers. But rarely is it ever a murder."
Chris Kyle Trial