Mass shootings, suicides, and psychoactive drugs.

If you want to go your own way, then you do not want to be dependent on drugs. That is, drugs you don't need to further your health. This is a tricky topic, because in the USA only MDs may prescribe medication or take patients off medication. There is no law forbidding patients to research their medication (not yet), so you should read up on anything and everything you are taking and consult your MD regarding your findings.

School / shopping mall / post office shooting. 
So many of those mass killers were taking some selective serotonin reuptake inhibitors drug (SSRI) that it's almost impossible to find exceptions. If you are taking one of those drugs or know someone who is, then study the subject. Here is one of 30 thousand sites I found on my first search:

SSRI Stories http://www.ssristories.org/

 

SSRIs! It's not just mass murder, it's suicide too! Two side effects for the price of one! 

Robin Williams' death highlights challenge of treating severe depression

CBS News | August 12, 2014

The U.S. Food and Drug Administration warns consumers that people who take SSRIs may have an increased risk for attempting suicide.

http://www.cbsnews.com/news/robin-williams-death-highlights-challenge-of-treating-severe-depression/

Shamed Glaxo's u-turn on 'suicide' drug

Daily Mail | 15 June 2004

http://www.dailymail.co.uk/health/article-306667/Shamed-Glaxos-u-turn-suicide-drug.html