Radak: This is said in astonishment.

Malbim: Surely, he will not live; Zechus Avos will not help him, since he did all abominations. This refers to life of the Nefesh in the world to come.

Radak: Damav is plural, and Yihyeh is singular, for Dam is mentioned in the Klal and the Prat. An expression of Havayah applies, like "Ki Yihyeh Na'arah" (Devarim 22:23).

Malbim: Also in this world he will be punished bi'Ydei Adam. He is Chayav Skilah for idolatry, and Chayav Chenek; one who is liable two different deaths, he receives the harsher one ¹ (Sanhedrin 37b).