David

He said to his men, "The LORD forbid that I should do this thing to my lord, the LORD’s anointed, to put out my hand against him, seeing he is the LORD’s anointed." (1 Samuel 24:6 ESV)
But David said to Abishai, "Do not destroy him, for who can put out his hand against the LORD’s anointed and be guiltless?" (1 Samuel 26:9 ESV)

Twice David was presented with the opportunity to kill Saul and seize the throne. Twice he refused to do it. Let us consider David's condition and all the reasons he could thought of to kill Saul:
Jobless - David was sacked by Saul;
Desperate - David was a fugitive pursued by Saul;
Opportunity - Saul was unguarded;
God's Will - God had anointed David as the next king.

Despite this, David refused to lay hands on Saul, preferring to let the Lord do the job.