from what i gather, viruses mutate to become less lethal, not more. the virus wants you to spread it around as much as possible, if it kills you off fast it doesn't get a chance to spread as widely as if it gives you only mild symptoms so you can continue to spread it, so it generally mutates to that end, ie higher infection rate, but lesser symptoms. we can but hope that this is the case with omicron. delta was already more contagious then the previous ones and had a lower mortality, now omicron is even more virulent and even less deadly.
yes, that's often true, but not always. there are a few examples in the link. spanish flu and ebola were two iirc