The Colts defense has been bad this year. They are towards the bottom in almost every major statistical category including total defense, pass defense, and run defense.
Most people would chalk this up to having bad players or having a bad scheme in place. Chuck Pagano was supposed to be a defensive mind when he was hired back in 2012, so many thought the defense would be much better than it currently is. Add that to the fact that Chris Ballard, the new Colts GM, spent a good part of the off-season upgrading the defense in terms of personnel, and the unit should be playing better.