[ryanlol]

Do you attribute a car bombing to the person that set up the bomb or the guy that inevitably turned on the car and set off the bomb?

In this case WannaCry creators built a system that would inevitably be triggered within a few hours from the malware going live.

I think it is meaningless to attribute the WannaCry killswitch to him instead of the authors. If he hadn't registered the domain some other threat intelligence firm would've done it moments later.