John Gill Commentary Judges 4:21

"Then Jael Heber`s wife took a tent-pin, and took a hammer in her hand, and went softly unto him, and smote the pin into his temples, and it pierced through into the ground; for he was in a deep sleep; so he swooned and died." — Judges 4:21 (ASV)

Then Jael, Heber's wife, took a nail of the tent
When she perceived he was fast asleep, and having an impulse upon her spirit to kill him, which she was persuaded, by its effect upon her, was of God; not filling her with malice and revenge, but a concern for the glory of God, the interest of religion, and the good of Israel, she took this method to effect the death of this enemy of God, and his people; having no arms in the house, for the Kenites used none, she took up an iron pin, with which her tent was fastened to the ground:

and took a hammer in her hand ;
which perhaps she knew full well how to handle, being used to driving the pins of the tents into the ground with it:

and went softly unto him ;
lest she awake him

and smote the nail into his temples :
as he lay on one side, these being the tenderest part of the head, from which they have their name in the Hebrew language, and into which therefore a nail, or iron pin, might be more easily driven:

and fastened it into the ground ;
she smote the nail with such force and violence, that she drove it through both his temples into the ground on which he lay; and then, as it seems, from (Judges 5:26) ; cut off his head, to make sure work of it:

for he was fast asleep and weary ;
and so heard not; when she came to him:

so he died ;
not in the field of battle, but in a tent; not by the sword, but by a nail; not by the hand of a man, but of a woman, as Deborah foretold, (Judges 4:9) .