This praise of Jael, despite her seemingly treacherous assassination of Sisera as he slept, is warranted in Deborah's song. For twenty years, Sisera had mightily oppressed the Israelites. The Lord had also long ago commanded the destruction of these Canaanites. Furthermore, Sisera and his men would each have taken "to every man a damsel or two" for his own sport if they had prevailed (Sisera would no doubt have abused Jael, given the opportunity), as even Sisera's mother gloated (Judges 5:29,30).

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8