Many muslim cultures have practiced cousin intermarriage for so long that they have very high rates of genetic defects and heritable disease, including mental illness.
Jihadi teachings conveniently redirect the violently mentally ill towards outsiders and remove them from the gene pool, thus protecting the tribe.
I should also point out that current cultural memes in the US, brought to us by the liberal elite ensconced in our educational system has a tendency to direct the violently mentally ill inward, which explains why schools and malls are frequent choices for the mass homicidal impulse.