He Prays That God Will Deal With His Treacherous Friend, Along With His Associates, By Death (Psalms 55:15).
He prays for sudden death to come on these men. The fact that David prayed like this indicates that the man's betrayal had been so serious that it had endangered his life. He had been so treacherous that he had sought David's death whilst David still trusted him as a friend. And thus David prays that God will cause the behaviour of his treacherous friend and of his friend's associates, to rebound on them. He is basically praying that, in the same way as they have tried to sow death for him, they themselves will reap death. Let them receive what they deserve.
‘Let death come suddenly on them,
Let them go down alive into Sheol,
For wickedness is in their dwelling,
In the midst of them.'
He prays that just as these men have sought his life, death may come suddenly on them. The violence of his expression confirms that he saw what they had done as unforgivable. He sees them as having been acting vindictively. So he prays that they may go down alive into the grave world, Sheol.
The prayer that they might go down alive into Sheol, the grave world, possibly has in mind the fate of Korah and his company in Numbers 16:30-33. They too had been treacherous, and had acted against God's chosen ones (the Aaronide priests), and they were described as being swallowed up alive by the ground and as going down into the Pit. This confirms that David saw these men, led by his one time friend, as treacherous in the extreme, and therefore deserving of the worst of fates. It is quite possible that not many had been willing to act against a respected commander like David, and that Saul had therefore had to seek out such as would betray him. And all this is confirmed by his reference to wickedness as something that was in their dwelling, and even in their inmost hearts. He saw them as enveloped in wickedness. Their behaviour had appalled him, and cut him to the heart.