It doesn't intentionally choose how it mutates, it just mutates randomly. Some viruses mutate more than others. It is by chance that the more infectious strains spread more often. And it is by chance that a more infectious strain may be more deadly. Some virus strains are so deadly, they essentially kill everyone off rapidly and the virus itself goes away. Some are so benign (common cold) that they linger forever. Look at Ebola or rabies - highly deadly, but not terribly contagious, as you either have to get bitten or get infected fluid in you. They kill quickly, so they have the potential to kill off a population. Hepatitis C on the other hand is highly deadly as well, but it takes a long time to slowly kill your liver, which eventually leads to killing you. During this time you can continue to spread it to others.

It's all natural selection. The virus isn't trying to do anything. It all just happens by chance spontaneous mutations and chance infections and propagation between people. If a virus kills its host before it has a chance to spread, it won't be successful. If it spreads quickly, then it doesn't really matter if it later kills the host.

Because the Chinese made it that way.

It is true that the natural selection of viral mutations is to become LESS deadly but more contagious. This is the case with the delta variant. A recent British study showed a mortality rate of .9% with the delta variant c/w 1.9% of native covid. I believe there have been 2 instances of viral mutations leading to increased lethality: the 1918 Spanish Flu (presumptive) and 2009 H1N1.