‘How long will you set upon a man,
That you may slay him, all of you,
Like a leaning wall,
Like a tottering fence?'
But while he sees God as his Rock and his strong tower, He is invisible to his adversaries who consequently see him as vulnerable and collapsing. They see him as like a leaning wall which could fall at any moment, and as a tottering fence which is totally insecure. They do not realise that God is with him. That is why they are going about to slay him. They do not realise how foolish their attitude is when they are dealing with one with whom God is pleased.
So David asks them how long they intend their behaviour and attitude to go on? For how long are they futilely going to set on him in order to slay him? He has no fear, for his confidence is in God.