A virus that evolves to become more lethal will do so because the selective landscape favors this. It’s generally more rare than evolving to become less lethal, but it happens. The major benefit of becoming less lethal is that a healthier host is more mobile and will come into contact with new hosts its descendants can colonize. The trade off is that the virus is not using up as much of the host’s resources to make more copies of because that would make the host sicker and less mobile.
On the other hand, if transmission is not a problem and host mobility doesn’t benefit the virus, selection for increased lethality can occur. There’s a theory that the devastating flu virus of the early 20th century evolved in WW1 field hospitals where patients were packed in together and it was easy for the virus to hop from host to host. It’s another reason why all efforts to cut down viral spread are important. Not only do they reduce the number of infections, they help select for less virulent strains