Summary: Barzillai ventured & volunteered to help king David during the most darkest moment of David's life, while David was running away from his own son Absalom.
2 SAMUEL 19:31-39 [The Life of David]
THE KING REWARDS THE FAITHFUL
David has sinned by committing adultery with Bathsheba and by having her husband killed. God has indicted David for his sin through Nathan the prophet, and David has repented. Nevertheless, there are certain consequences he has had to face. There was the rape of his daughter Tamar by her half-brother, Amnon. Then there was the murder of Amnon by his half-brother, Absalom. Absalom fled to Geshur, where he was given sanctuary by his grandfather, King Talmai. Through the intervention and intrigue of Joab, David was pressured into allowing Absalom to return to Jerusalem. In time, Absalom succeeded in undermining the reign of his father, David. Absalom revolted against his father and sought to take over kingdom. Absalom's revolution forced David, his family, and his supporters to flee to the wilderness. God spared David, gave his army victory over the rebel forces. 2 Samuel 18:9 tell us Absalom died in an extremely unusual way, through an accident where his head providentially was caught in the branches of a tree while riding his mule. Joab, the general of David's armies, then ran him through with a javelin while Absalom was hanging in this tree, in spite of David's specific orders not to harm him.
Chapter 19 tells of David's return to Jerusalem after a forced exile caused by Absalom his son. David's son Absalom is dead now and his army has been defeated. So King David is returning to Jerusalem to retake his rightful place on the throne-Israel.
David's return finds the people of Israel in various states of mind and in various states of preparedness for his return. In this chapter, [and in the first verses of chapter 20,] we are presented with five different people who met the king. Each one meets him with a different attitude & from a different point of view. Needless to say, not everyone was happy to see the king.
[I. The King Greets Different persons]
The 1st & 2nd persons to met the king are found in verses16-23. Ziba, the one time chief servant of Saul, & then there was Shimei who threw stones at David. Both were worried about meeting the King. Shimei repents and is forgiven
The 3rd is in verses 24-30. Mephibosheth came out to meet the king. He was waiting for the king. He had been misrepresented by his servant Ziba. Mephibosheth hadn't washed or shaved since the day David left Jerusalem. He had apparently spent his time in mourning over the king's absence & in waiting for his return!
These verses turn from people whose motives were questionable to the 5th person who was the wicked man Sheba in 20:1-3. He was angry and rebellious over the return of the king.
[II. The King Greets Faithful Barzillai, 31-39]
Our lesson tonight though is about the 4th man to meet the returning King in verses 31-39. His name is Barzillai. I would guess that not many have heard about this wonderful man named ‘Barzillai.' Have you? He ventured and volunteered to help king David during the most darkest moment of David's life, while David was running away from his own son Absalom. David is broken-hearted and tired, running to save his life from his own son Absalom. Let's read 2 Samuel 17:16 to understand the background. "Now therefore, send quickly and tell David, saying, 'Do not spend this night in the plains of the wilderness, but speedily cross over, lest the king and all the people who are with him be swallowed up."
The enemy was ready to destroy the King and his people! After king David crossed Jordan and came to a place called Mahanaim, an elderly man called Barzillai and a few other men came with wagon loads of ‘goodies' to meet David who was weak, tired, depleted, miserable and in pain. 2 Samuel 17:28-29 list some of the supplies three land owners brought. "[they] brought beddings and basins, earthen vessels and wheat, barley and flour, parched grain and beans, lentils and parched seeds,  honey and curds, sheep and cheese of the herd, for David and the people who were with him to eat. For they said, "The people are hungry and weary and thirsty in the wilderness."
Don't miss the ‘beds' among the various other stuff! God knew David and his men were tired and they need some rest very badly. Barzillai knew that things had turn against David, with Absalom after David's life; yet he helped David. He was elderly and had all kinds of reasons to give excuses; yet he volunteered to help David.
Here again we find Barzillai as another person who came to meet David when he returned to Jerusalem. He meets the return of the king with great joy & happiness. Verses 31-32, "Now Barzillai the Gileadite had come down from Rogelim; and he went on to the Jordan with the king to escort him over the Jordan. 32 Now Barzillai was very old, being eighty years old; and he had sustained the king while he stayed at Mahanaim, for he was a very great man.