Final answer:
The question asks about a person killed by an FBI-agent in 1974 after 18 murders, but no references match this specific event in the provided materials. Historical documents mention various incidents involving FBI agents, but none correlate to both the year and the circumstances described in the question.
Step-by-step explanation:
The question appears to be seeking information about a historical event or person who was involved in a series of murders and was ultimately killed by an FBI-agent in 1974. However, there is no conclusive detail provided that matches the year 1974 with the killing of a person by an FBI agent after committing 18 murders. Various historical events such as the confrontation at Wounded Knee in 1973, the assassination of Robert F. Kennedy by Sirhan Sirhan in 1968, the assassination of Malcolm X in 1965, and the actions of undercover FBI agents inside the Nation of Islam are documented, but none fit the exact specifics of the question.
Please note, the details given for reference do not directly correlate with the death of an individual by an FBI agent in 1974 after 18 murders. Therefore, without further specification or clarification of the question, we are unable to provide a precise historical account that matches these criteria.