John Gill Commentary 1 Samuel 19

And Saul spoke to Jonathan his son
Who was heir to his crown; and though he knew he loved David, and was in strict friendship with him, yet he might hope, that as his succession to the kingdom was in danger, as he thought, and that David was his rival in it, his mind would be alienated from him; and that he would listen rather to a father than a friend, and would see where his true interest lay, and abandon David, yea, seek his ruin, which Saul was intent upon

and to all his servants ;
who yet pretended to love David, and as he himself said they did, and some of them might; with these he might rather hope to succeed, as they were attached to him, and might be secretly enemies of David, and therefore to these, as well as to his son, he spoke, and gave his orders:

that they should kill David ;
as if he was a traitor, and an usurper of his throne, and one that had a design upon that, and upon his life; finding he could do nothing by the schemes, and snares, and stratagems, he used in a private manner, he grew outrageous and furious, and openly declared his views, and laid his injunctions on his son and servants to take away David's life, as a very dangerous person to his crown and government.

But Jonathan, Saul's son, delighted much in David
In his company and conversation; he loved him with a love of complacency, and was constant and steadfast in it, and which was a kind providence to David; for by this means he came to the knowledge of Saul's designs upon him, and could the better guard against him:

and Jonathan told David, saying, Saul my father seeks to kill you ;
to inform him of which was acting the part of a sincere and faithful friend:

now therefore, I pray you, take heed of yourself until the morning :
it seems it was now evening when he informed him of it; and as he knew not what emissaries Saul might have out that night in quest of him, he advises him to take care of himself, and not expose himself to any danger, and to keep a strict guard about him; and in the morning he would try to conciliate his father to him, when he might hope, having slept upon it, that he would be in a better temper, and more disposed to hear what might be said to him:

and abide in a secret [place], and hide yourself ;
he seems to suggest as if it was not safe for him to be in his own house, and in his own bedchamber that night, but that it was advisable to retire to some private place, where it might not be known or suspected that he was there. By what follows he means some field, and a private place in it.

And I will go out
In the morning, at the same time his father used to take his morning walk:

and stand beside my father in the field where you [are] ;
on that side of him next to David, that he might not see him, and yet be so near, that David might hear what passed between them:

and I will commune with my father of you ;
speak in favour of him, and endeavour to dissuade him from attempting to take away his life, which was of so much importance and usefulness in the commonwealth of Israel:

and what I see that I will tell you ;
what David could not well hear he would inform him of, and what he could perceive in the countenance of Saul, as well as conclude from his words, that he would make known to David, that so he might know better what he had to do, and provide for his safety.

And Jonathan spoke good of David unto Saul his father
Observed to him what a good man he was, and what good things he had done, what wisdom and prudence he had shown in the management of all his affairs, what valour and courage in all his expeditions, what faithfulness and integrity to his king and country in every instance:

and said unto him, let not the king sin against his servant, against
David ;
by taking away his life; which would have been a great sin indeed, a sin against the law of God, which forbids murder, and which would have been attended with sad aggravations of cruelty and ingratitude:

because he has not sinned against you ;
had not disobeyed any of his orders, but faithfully served him in everything, and much less ever thought to take away his life, or seize his crown, as he might imagine:

and because his works [have been] towards you very good ;
by slaying the Philistines, when he and his army were in the utmost terror; by driving away the evil spirit from him, through playing on his harp before him; as well as by commanding his troops, and leading them against the Philistines, and obtaining victory over them.

For he did put his life in his hand
Exposed himself to the utmost danger, when no one in all the camp of Israel would do the like:

and slew the Philistine ;
Goliath of Gath, who defied the armies of Israel; against him he went unarmed, only with his sling and stones, and fought him, and slew him:

and the Lord wrought a great salvation for all Israel ;
by his hand, so that they were delivered from their enemies, who fled before them, and they pursued them, and got a complete victory over them:

you sawest [it], and didst rejoice ;
he was an eyewitness of David's going forth against the Philistine, and slaying him, and of all the happy effects of it, which then greatly affected him, and he could not forbear expressing great joy on that occasion:

wherefore then wilt you sin against innocent blood, to slay David
without a cause ?
and so entail the guilt of it on himself and family.