It could be an intermediate version - designed to test contagion and immune response before a final, more lethal variant was produced. It escaped the Wuhan lab due to incompetence or malice.
That doesn’t make any sense. The SARS coronavirus IS already much more lethal. A bioweapons engineer would start with the most lethal virus and work to make it more lethal and contagious.
If you were attempting to make an ethnic-specific plague, you would start with a mild virus and work out the targeting genes in test versions. Then you would enhance the lethal potential with additional modifications to make the finished version.
2019-nCoV looks to me like an early research variant that got out of control due to a lab accident.
The swiftness and unconcealed severity of the Chinese Government reaction is the big "tell" here. You do not shut down the core of your national economy, quarantine 75 million people, and isolate the capitol city for a new seasonal flu that causes a couple hundred deaths.