Evolution dictates viruses evolve up in virulence and down in lethality. A virus measures fitness by the ability to spread and not kill its host too soon, or ideally not kill it at all and spread to everyone and everything. More effective transmission is the evolutionary goal. If this thing is not mutating towards more virulence it is fighting the current that is its evolutionary goal. Artificial means may have been used to maximize this things virulence whether it be CRISPR or artificial mutation/selection for virulence. (ie. Breed a dachshund with a great dane and breed their offspring repeatedly and you get a dog that looks like the Iraqi mutts. Artificial selection breeds out.)
Yet another check mark in the column this was lab created.