Simple answer, tough problem

Why are most rampage shooters male?

Simple answer: because they are more likely to have antisocial mental illness and to project it out into the world:

[ American Psychological Association ] When it comes to mental illness, the sexes are different: Women are more likely to be diagnosed with anxiety or depression, while men tend toward substance abuse or antisocial disorders, according to a new study published by the American Psychological Association. Published online in APA’s Journal of Abnormal Psychology, the study looked at the prevalence by gender of different types of common mental illnesses. The researchers also found that women with anxiety disorders are more likely to internalize emotions, which typically results in withdrawal, loneliness and depression.

Men, on the other hand, are more likely to externalize emotions, which leads to aggressive, impulsive, coercive and noncompliant behavior, according to the study.  The researchers demonstrated that it was differences in these liabilities to internalize and to externalize that accounted for gender differences in prevalence rates of many mental disorders.