וַיֹּסִ֙פוּ֙ בְּנֵ֣י יִשְׂרָאֵ֔ל לַעֲשׂ֥וֹת הָרַ֖ע בְּעֵינֵ֣י יְהוָ֑ה וְאֵה֖וּד מֵֽת׃
And the children of Israel again did that which was evil in the sight of the LORD, when Ehud was dead.
Metzudat David on Judges
it wants to teach that from the day that Ehud died, and even during the days of Shmagar, they did evil. And its for this reason that his salvation was not a big one