Viruses, like all things, mutate to become more fit to their environment. A lot of viruses mutate to become less deadly because they're killing their hosts too quickly, and the less-deadly versions allow their hosts to live longer, and thus spread better.

COVID already spreads very well, what with the ability for it to spread while incubating silently for a couple weeks. A more-deadly version that retains the slow and silent incubation, but then kills mercilessly, would likely still spread just as fine.