Bodyguards are supposed to provide security, but it is important to screen their motivation, and personal grudges before selecting a bodyguard. This is because they have access to a lot of confidential information about the person they are supposed to protect, and they can misuse that information for personal benefit or to even harm the person they are supposed to protect.
A classic example is the former prime minister of India, Indira Gandhi, who was shot dead by her bodyguards in 1984. There are few people who have so many enemies, especially ordinary civilians that they need surveillance or security. In many cases, the surveillance is wasted on harmless civilians who would not harm even a fly, instead of the real troublemakers who remain undetected. A month or two is sufficient to understand the character and motivation of a person, anything longer is a breach of privacy and human rights, especially for a person who has not asked for the security.