What's the benefit for a pathogen to be more deadly? Killing the host is actually bad for it, since that ends spreading (with this host at least).
Our assumptions about evolution is that its driven by the need to survive. When a pathogen is faced with a change in their environment bought on by a pharmaceutical treatment it is possible for a pathogen to adapt to fight or avoid that treatment. This does not mean they'll automatically adapt (they're not the Borg), in fact in most cases the opposite can occur where the pathogen is completely (or nearly completely) wiped out such as the case of smallpox.
Evolution does not consider risks and benefits, changes are random. Sometimes these changes can cause a species to die out by destroying its environment. As for pathogens, killing the host is often required for a pathogen to spread, especially for pathogens that only spread through direct contact (not air, food or water borne), not killing the host by producing symptoms that allows the pathogen to spread is going to result in the pathogen dying out.
In nature, when there is a rapid change in the environment, most species end up dying. This is the bad part of anti-biotics as its a similar event on a mirco scale. They're indiscriminate, so they'll kill the micro-organisms in our body that aren't just benign but helpful. For this reason alone we should try not to not over subscribe anti-biotics, however anti-biotic resistant pathogens are also a consideration.