Nothing is impossible to us humans, from inception till date, no accurate prediction about the nature of a person can say exactly what a person can or cannot do. If humans are capable of creating a deadly virus? i'd say why not...is it not the same humans that created the deadly nuclear weapons and other weapons capable of mass destruction for the purpose of killing, nothing stops them from engineering a virus for same purpose as well. Humans are more than capable of creating a deadly virus to kill.
We can't exactly say that this is true or can't also deny this theory. To some extent, this can be possible but for a broader look, this theory seems to be unrealistic.
A lot of people say that these new diseases are a part of biowarfare but no one has any evidence to prove their theory.
The thing is that lots of people are dying due to these diseases so instead of focusing on these theories we batter be focused on the treatment and preventive measurements.