In And Then There Were None why does Dr Armstrong disappear?

He wanted to create the most unsolvable, greatest, mystery death that nobody could solve. He had been a judge for so long and he wanted to do something bad. He had a craving to kill, so he decided to make this scheme and make a billiant death of 10 people tha would be hard to solve, until someone found his letter in the bottle...basically, he had a craving to kill and he wanted it to be a really big, unsolvable death, so he did it according to the poem of the ten little indian boys