I have no idea what exactly this thread is reffering to since I haven't heard anything about the DC sniper case in years but I had always thought that the older guy, John Allen Muhammad, was the psychopath suffering from delusions and he was able to convince the child, because of his implicit position of authority over him, to carry out these attacks. Muhammad was suffering from severe mental illness but from what I understand Lee Boyd Malvo wasn't crazy per se, just confused because of the influence of Muhammad. Kind of like how a child raised and indoctrinated in any kind of cult will act on those beliefs. I wouldn't be surprised if the same power dynamic was in play with the Columbine shootings since the two perpetrators were very close friends.
Mental illness
is a factor in a lot of crimes that aren't drug-related or a "crime of passion." Statistically speaking, most violent crimes aren't random acts of violence except in cases involving someone suffering from mental illness (serial killers, mass shooters, etc). What you need to understand is that mental illness isn't a death sentence. In most cases it can be treated with proper help or in severe cases, neutralized before it becomes an even bigger problem for society at large. Psychiatry is a badly underfunded field of medicine and study because insurance companies don't pay well for any medical specialty that doesn't involve procedures and operations (psychiatry requires a lot of time and doesn't if ever involve any procedures - both these things aren't in the interests of insurance companies since it's hard to prove that all the time in therapy is making a difference; it's not something that is measurable using objective metrics) and the general public seems to have a hard time accepting that mental illness exists in the first place (i.e. this thread, and the entirety of the religion known as Scientology). The fact that there is an anti-vaccine movement in the United States should tell you how distrusting the general publc is towards medicine in general and how susceptible they are to anything that exploits those doubts (i.e. see homeopathic medicine).
What you also have to understand is that mental illness isn't an excuse to behave badly hence why John Allen Muhammad was sentenced and executed. The issues with dignosing and treating mental illness are a lot more nuanced and complex than you think they are OP.
So wanting to kill people is a mentall illness now?
Anti-social behaviors are broadly associated with mental illness but that isn't to say that being anti-social is itself a mental illness.
PS. sorry for any spelling errors, typing on a tablet... :\
FOR MURDEROUS CAC'S. Mental illness may be a real issue but that doesn't absolve a motherfukker of their murderous crimes. Why is that defense only brought up when cac's go on a rampage? 
you wouldnt be sayin this shyt if he shot your dad while he was pumpin his gas. fukk outta here