Matthew Poole and continuators on 2 Kings 21:16

16 Moreover, Manasseh shed innocent blood very much, till he had filled Jerusalem from one end to another, beside his sin wherewith he made Judah to sin, in doing that which was evil in the sight of the LORD.

The Blood of those Prophets and Righteous Men who either reproved his sinful practises, or refused to comply with his wicked commands and worship.

i. e. His Idolatry, which is elsewhere called evil and corruption, and here sin, by way of emmency; which is the more considerable, because it is here compared with horrid cruelty, and implied to be worse than that, and more abominable in Gods sight, because it doth more directly and immediately strike at the Glory and Purity of the Divine Majesty, by respect unto which all sins are to be measured. And this expression God here useth in opposition to the gross error of most men, who look upon Idolatry as a small sin, as a meer mistake of the mind, as the fruit of a good intention, and as an excess proceeding from Zeal in Religion.

Marginal references: Heb. from mouth to mouth.