No political motive apparently, just sociopathy plus firearms.
Psychopathy, more or less, is the clinical term for sociopathy, and the two are often used interchangeably.
sociopathy so·ci·op·a·thy (sō'sē-ŏp'ə-thē, -shē-)
The behavioral pattern exhibited by sociopaths.
sociopath so·ci·o·path (sō'sē-ə-pāth', -shē-)
A person affected with an antisocial personality disorder.
Someone whose social behavior is extremely abnormal. Sociopaths are interested only in their personal needs and desires, without concern for the effects of their behavior on others. (Compare psychopath.)