Q. I read in the Bible many years ago that God told one man (I forgot who) that he would be king, and the man replied that there was already a king, so how could he become the king? God answered him something like I will help you kill him so you can become the king. Where in the Bible do I find this?
The story line you’re describing actually sounds to me a lot more like Shakespeare’s play Macbeth than something in the Bible. Near the beginning of that play, three witches greet Macbeth as “thane of Glamis” (his actual title), as “thane of Cawdor” (a position that belongs to someone else), and as someone who will “be king hereafter.” Macbeth replies:
I know I am thane of Glamis;
But how of Cawdor? the thane of Cawdor lives,
A prosperous gentleman; and to be king
Stands not within the prospect of belief,
No more than to be Cawdor.
In other words, Macbeth knows he has inherited his own title, but there is already a thane of Cawdor, and there is already a king, so those titles can’t apply to him. But right afterwards, he learns that the king has executed the thane of Cawdor as a traitor and given him his title. Unfortunately Macbeth reasons that if the first thing the witches told him was true, then the second could be true as well, and he might become king. Even though his friend Banquo warns him that “to win us to our harm, the instruments of darkness tell us truths” (in other words, the witches have no good intentions), Macbeth unfortunately decides he should kill the king and take over the throne, since that has been predicted for him. This leads only to disaster for Macbeth—but I won’t tell you any more here; the play is so good, you really need to see it or read it for yourself!
The one place in the Bible that’s somewhat similar to what you’re describing is where God tells David that he will become king, even though a different man, Saul, is currently on the throne. But God never tells David that he will help him kill Saul. And although David later gets two opportunities when he can easily kill Saul and even claim that this is in self-defense, he does not. Instead, despite encouragement from his own soldiers that God has provided these opportunities in order to fulfill His word that David will be king, he replies (swearing an oath):
“As surely as the Lord lives, the Lord himself will strike him, or his time will come and he will die, or he will go into battle and perish. But the Lord forbid that I should lay a hand on the Lord’s anointed.”
That is, David expected that Saul would either die of some illness or accident, or in battle, or even of old age, but he wouldn’t do anything to kill Saul himself, even though God had promised him that he would be king. So you may be remembering what God predicted for David while Saul was still alive, but perhaps mixing that memory with some details from another story.