I'd probably say Parkland. I'd say that really got me interested in mass murderers. Columbine would be a close second though, because I researched that a little before Parkland.
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"Now! Feel death, not just mocking you. Not just stalking you but inside of you. Wriggle and writhe. Feel smaller beneath my might. Seizure in the Pestilence that is my scythe. Die, all of you."
- T.J. Lane (in his Facebook poem)