John Gill Commentary 1 Kings 1:51

"And it was told Solomon, saying, Behold, Adonijah feareth king Solomon; for, lo, he hath laid hold on the horns of the altar, saying, Let king Solomon swear unto me first that he will not slay his servant with the sword." — 1 Kings 1:51 (ASV)

And it was told Solomon
By some of his courtiers:

saying, behold, Adonijah fears King Solomon ;
lest he should take away his life:

for, lo, he has caught hold on the horns of the altar ;
which was the last resort of the guilty when they despaired of mercy otherwise:

saying, let King Solomon swear unto me this day that he will not kill
his servant with the sword ;
he owns Solomon to be king, and himself his subject and servant; this no doubt he did to conciliate his favour, nor did he think his life safe, unless Solomon promised with an oath, that he would not take it away.