Walter Pierce was a Starfleet engineer. He murdered his girlfriend and her lover in a jealous rage and then killed himself by jumping into the plasma stream of the Enterprise-D.
Walter was part Betazoid, and this left empathic traces in the nacelle tube bulkhead. These traces effected others and one crew member, Daniel Kwan, killed himself. While investigating his death, Deanna Troi was effected also but saved at the last moment. (TNG: "Eye of the Beholder")