Wow - this sounds stranger than fiction!

Hutchins and Hankins knew if the kill switch went down, the malware
would pick up where it left off, infecting thousands of computers every
minute. Puffy eyed and sleep deprived, they knew the domain had to stay
up at all costs. The researchers fended off several attacks from an
angry operator of a botnet trying to knock the domain offline with junk
internet traffic. And, at one point, law enforcement seized two of their
servers from a datacenter in France amid confusion that the domain was
helping to spread WannaCry and not preventing it.

It's almost like they captured an evil genie in a bottle - and have to forever watch the bottle in case it comes loose!