Spagnuolo was historically awful in his one season as the Saints DC (2012). Only team in the history of the league to give up 7,000 yards in a season. The offense was top-3, with Brees, J. Graham, Sproles, Ingram, and an excellent O-line. But every game was a shootout, the kind of football that is frustrating to watch - win or lose. The only team they held under 300 yds was the Falcons (283), and they lost because of five turnovers.
That defense had decent, though aging, talent with Jon Vilma, Will Smith, Roman Harper, and Jabari Greer (all over 30) plus a handful of good younger players (Cam Jordan, Malcom Jenkins, Akiem Hicks). But they couldn't get off the field. Third and long; no problem - TE quick slant was an automatic 8 yds. Must have seen it 40 times that season. Coaches never found a way to scheme around a clear weakness, and every opponent exploited it. By the end of the season, the defense just gave up, yielding 530 yds to Carolina in the final game to set the record. Poor performance, yes, but it's also a sign of poor coaching when you can't motivate a veteran team to avoid being the worst ever.
It's great that the Chiefs are playing good defense this season, and I wish them the best. But if you are going to give Spagnuolo credit for that, then you've got to give him blame for coaching the worst defense in history. Until somebody breaks that record, he owns it.