Trobec was born as an illegitimate child in Planina nad Horjulom. At the age of 14 he is said to have burned his neighbor's hay, and to have stolen a moped at age 18. After doing his military service he worked for Yugoslav Railways and then emigrated to Germany. He returned to Slovenia in 1974 and committed several crimes, and was sentenced to 13 months in prison. Between 1976 and 1978 he raped, killed, and cremated at least five women[Note 1] at his home in Dolenja Vas pri Polhovem Gradcu. He was sentenced to death (the last death sentence in Slovenia), but this was commuted to 20 years in prison. During his imprisonment he tried to kill fellow prisoners twice, and 15 years were added to his sentence. He committed suicide at Dob pri Mirni Prison in Slovenska Vas on 20 May 2006. He was buried at state expense in an anonymous grave in Šentrupert. At the time of his death he had prostate cancer and spent 27 years in prison, which is a Slovenian record.
^Trobec was convicted of the murders of Vida Markovčič, Marjana Cankar, Urška Brečko, Ana Plevnik, and Zorka Nikolić. After his sentencing, the identity papers of Olga Pajić—also presumed killed by Trobec—were also found in his house.