Saul said to Samuel, I have sinned; for I have transgressed the commandment of Yahweh, and your words, because I feared the people, and obeyed their voice.
(2) Another possibility is that Saul might have threatened to take Samuel's life if he refused.  His seizing Samuel's robe was in itself an act of violence; and Saul was certainly capable of killing anyone whom he considered to be a threat to himself.