As others have noted, our tendency to use "want" or "designed" around evolution tends to lead us to more intention from the process than is factual.

For viruses the primary issue is R0 ("R naught"), which is the number of people who become infected for every infected person, so an R0 of 2 means that if I catch that virus I will infect (on average) two people. Anything over 1 means that the virus will spread to more people for each generation of spread.

A virus that is highly contagious will have a high R0 value. If a virus has a high R0 and also kills people after they spread the virus, that's irrelevant for the virus because it has already spread. The severity of the virus will often correlate with its contagiousness because the symptoms go along with the method of spread -- for example, respiratory viruses make people cough, which spreads the virus in the air, and a particularly virulent respiratory virus may make someone cough/spread a lot and also kill the host. If a particular virus evolves to be "worse," and that means it spreads to four people instead of 2 and also kills the host in 20% of cases, that virus will still spread faster than the old virus, even if the host is now dead.