And he would know it is an illegal order how? He should just wait and see if the Chinese/NK nukes hit the US first before deciding to follow the orders?

His answer was the right one. Just because you and I don't like Trump doesn't automatically make any order he gives to the military Illegal nor does it give the military commanders the right to ignore them.

Not only that but a US Admiral claiming he would not follow orders and launch an attack if ordered puts the entire MAD principal in jeopardy.