but thou shalt surely kill him; thy hand shall be first upon him to put him to death, and afterwards the hand of all the people.

Instead, you must surely kill him. Your hand must be the first against him to put him to death, and then the hands of all the people.

`But thou dost surely kill him; thy hand is on him, in the first place, to put him to death, and the hand of all the people last;

but thou shalt in any case kill him: thy hand shall be the first against him to put him to death, and afterwards the hands of all the people;