John Gill Commentary 1 Kings 2:37

"For on the day thou goest out, and passest over the brook Kidron, know thou for certain that thou shalt surely die: thy blood shall be upon thine own head." — 1 Kings 2:37 (ASV)

For it shall be, [that] on the day you go out
Out of the city of Jerusalem: and pass over the brook Kidron: which is particularly mentioned, because this lay in his way to Bahurim, his native place; he must cross that to go to it, see (2 Samuel 15:23) (16:5) ; and where it might reasonably be supposed he would some time or another be inclined to go, through business, or a desire to see it again:

you shall know for certain that you shall surely die :
it may be depended on as what will be most certainly the case; no reprieve nor pardon will be granted:

your blood shall be on your own head ;
fair warning being given, he could blame none but himself, should he be guilty and suffer.