I guess not everyone was happy that he stopped Wannacry. US agencies in particular.
Registering unregistered domains the malware connects to is a very obvious thing to do, in this case he got "lucky".
Just because someone else could have stopped it, doesn't mean he didn't stop it. That's... just a fact...
This was inevitable because of how WannaCry was designed.
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.