A subscriber to the extreme far right politics in Italy, Semerari also had close ties to the Banda della Magliana, whose meetings were usually organized by him in his villa at Rieti in the summer of 1978, through his group Costruiamo l'azione. He was also a member of Propaganda Due (P2) masonic lodge and maintained links with the SISMI, the Italian military intelligence agency, which allowed him to transfer information to the Banda della Magliana.
In exchange for financial support for his organization, Aldo Semerari would usually offer psychiatric evaluations to members of various criminal organizations tailored to the frequent arrests, which members usually underwent. In addition to the Banda sella Magliana, he had made the same deal with Raffaele Cutolo's Nuova Camorra Organizzata (NCO), as well as with the rival organization of the Cutolo, the Nuova Famiglia (NF) headed by Carmine Alfieri. Semerari helped Camorra boss Umberto Ammaturo, allied with the NF, to escape prison feigning insanity.
Aldo Semerari was eventually assassinated by the Camorra on 1 April 1981. His body was found decapitated. A criminal named Lugaresi Capemuorto claimed that Semerari was assassinated by the SISMI in order to prevent him from revealing what he knew about the 1980 Bologna massacre. He claimed that on 25 March 1981, he had received a telephone call in which Semerari begged for help because he was in Naples and in danger of being murdered. Then he added: "On March 25, 1981, I immediately called General Santovito ... warning him of the situation. Santovito, who certainly knew Semerari, was totally unsurprised and said: 'Keep the news to yourself', in the sense that I should not tell others." The magistrate, Rosario Priore, believed Capomuorto's allegations and developed a complex reconstruction of the murder and the alleged facts, but in the end, the theory was proved to be completely unfounded.
In 1982, Camorra boss Umberto Ammaturo and his lover, Pupetta Maresca, were arrested and charged with Aldo Semerari's murder. Ammaturo managed to escaped to Africa and then to South America, while Maresca remained in Italy to face the charges. She would serve four years in prison between 1982–86, before she and Ammaturo were acquitted on appeal in 1989 for lack of evidence. Ammaturo confessed the murder when he decided to become a pentito in June 1993. Maresca continues to deny any involvement in the murder.
While free and provided with a new identity in exchange for his testimonies, Ammaturo admitted to killing Semerari and personally decapitating him in a rare interview with La Repubblica newspaper in May 2010. Ammaturo considered Semerari to be a traitor siding with the enemy NCO in the fierce criminal war, when he provided Cutolo with a false appraisal report.
- (Italian) Il boss Ammaturo confessa: 40 avvisi, Corriere della Sera, 24 May 1994
- Behan, The Camorra, pp. 103
- (Italian) L'"Organizzazione ODESSA"
- Allum, The Neapolitan Camorra, p. 149
- (Italian) "Tagliai io la testa a Semerari; aveva tradito un nostro accordo", La Repubblica, 25 May 2010
- Allum, Felia Skyle (2000), The Neapolitan Camorra: Crime and politics in post-war Naples (1950-92), Brunel University
- Behan, Tom (1996). The Camorra, London: Routledge, ISBN 0-415-09987-0