From an evolutionary perspective, it is beneficial for a pathogen to work without killing the host because it allows the pathogen to continue to spread and infect new hosts. If a pathogen kills its host too quickly, it will not have the opportunity to spread to new hosts, and the pathogen will eventually die out.
By contrast, if a pathogen can infect a host and replicate within the host without causing too much harm, it can continue to spread to new hosts, increasing its chances of survival and propagation. This is why many pathogens have evolved to cause relatively mild symptoms or to be asymptomatic, allowing them to spread from host to host without being detected.
Of course, this does not mean that all pathogens are harmless or that they do not cause serious harm to their hosts. Some pathogens, like the Spanish Flu, can cause severe illness and death, and can be very difficult to control. However, from an evolutionary perspective, it is generally advantageous for a pathogen to work without killing the host, as this allows it to spread more effectively and persist in the population over time.