‘Authority’ – compare ‘authorisation’ and ‘written permission’. I expect it’s from former times when only a privileged minority could read, and even less could write.
Beware argument based mainly on motive. Everyone has motives, making it too broad. It’s more useful for eliminating a suspect because they don’t have any motive. I certainly have a motive to want a vaccine for covid-19; it’s a horrible and dangerous illness.
It’s not that “we’re all going to die”; the infection fatality rate is looking like 1% to 1.5%, concentrated towards the oldest in the population. It’s that it’d be entirely inhumane to let hundreds of thousands suffocate for a week until most of them die.