Being a novel (new) Corona-virus, nobody actually knows the extent of possible mutations. This is a strong argument against it being a weapon. However, it's impossible to contain everywhere, though we should perhaps make the attempt.

My understanding is that lethality and transmisability are typically a one or the other type situation. Viruses like HIV and to some extent COVID that kill their host effectively but slowly are rare. The typical case is to either multiple quickly inside one host (i.e. kill the host quickly) or move quickly from host to host (i.e. spread rapidly without killing the host because you need the host to be a vector)