Are you suggesting that admins are never responsible for what they say because every admin is a single individual?
Not at all. But by defederating the instance, you're not just punishing the admin, but every other user on that instance. It's a complete overkill reaction to something that is honestly a harmless threat. If I'd go nuts like that every time someone tells me he wants me to get killed I'd have a very high blood pressure.
Defederation is a last resort reaction if an instance is seriously incompatible with the rest of society, like .ml