That would be against the rules of evolution. A virus that kills too many of its hosts destroys its livelihood. Mutations are always around and they usually lead to less dangerous viruses.
Evolution has no rules. It’s pure random, the advantageous mutations has greater chance to pass on their genes while disadvantageous mutations has less chance. The only deciding factor is time.
Covid19 already has a couple of huge evolutionary advantages that gives it massive leeway in mutations - its long incubation period and it’s asymptotic transmission capabilities means high lethality doesn’t really present too much of an issue for it. It doesn’t matter if it kills more of its hosts if they have already passed on the virus by the time they die.
Indeed, some of the most deadly and horrible natural diseases in history have similar attributes- smallpox, the Black Death, they all had far higher lethality, which means COVID19 can afford to become massively more deadly before it impacts on its own viability.