Its hard to argue that humans aren't the cause of megafaunal extinctions. Everywhere humans went on the planet, the megafauna disappeared soon afterwards. The time-scale of extinctions always follows human migrations.
My personal thoughts on global Holocene extinctions are that in almost all cases humans are directly and indirectly responsible (i.e. directly through hunting, indirectly through destruction of habitat [burning], introduction of disease [e.g. with dogs], introduction of novel predators [dogs again], etc). In some cases there are probably natural factors working in combination but which by themselves would not have resulted in extinctions.
