The second half of the chapter answers the question of who may kill, and it does so with a workshop image that is one of the most effective in the book.
There is always the one in charge of executions who does the killing. To do the killing in his place is to hew wood in place of the master carpenter. And one who hews wood in place of the master carpenter rarely escapes cutting his own hands.
The argument works on two levels at once. Read plainly, it is about jurisdiction: there is an officer whose function this is, and a ruler who kills outside that channel has usurped a role and will be injured by the tools. Read as the chapter's point, the one in charge of executions is not a human official at all but the ordinary process by which things come to an end, and a ruler who takes that work on has stepped into a position that was never vacant.
The book supports the second reading elsewhere. Chapter twenty-five puts heaven and the way above the king in its sequence. Chapter twenty-nine says the world is a sacred vessel and that acting on it spoils it. Chapter thirty-two says the way is nameless and nobody can make a subject of it.
The image of the amateur at the workbench is also exactly right for the argument. The danger is not punishment for presumption; it is the plain physical fact that an unskilled hand on an edged tool is injured by it, and that the injury is self-inflicted and immediate.