John Gill Commentary 1 Kings 20:36

"Then said he unto him, Because thou hast not obeyed the voice of Jehovah, behold, as soon as thou art departed from me, a lion shall slay thee. And as soon as he was departed from him, a lion found him, and slew him." — 1 Kings 20:36 (ASV)

Then said he to him, because you have not obeyed the voice
of the Lord
In not smiting him; which, if he was a prophet, he must know how great an evil it was to disregard or disobey what was said by a prophet in his name; he must be inexcusable:

behold, as soon as you are departed from me, a lion shall slay you .
And as soon as he was departed from him, a lion found him, and slew
him ;
which may seem severe, yet being an act of disobedience to the command of God, by a prophet of his, was punishable with death.