Before the people - i. e. openly and publicly. The Septuagint turns the original of the above words into a proper name, Keblaam, and makes him the actual assassin, but without much ground.
Related Commentaries of 2 Kings 15:10
2 Kings 15:10
10 And Shallum the son of Jabesh conspired against him, and smote him before the people, and slew him, and reigned in his stead.