INDIANAPOLIS — There are plenty of atrocities for fans to remember from the Green Bay Packers’ historic second-half collapse Sunday at Lucas Oil Stadium.

After all, it takes a total team effort to squander a 18-point halftime lead and suffer a 30-27 loss to a team — the Indianapolis Colts — that was 2-14 last season and experiencing a storm of emotions after learning Monday that their coach, Chuck Pagano, is being treated for leukemia.

Let’s see, there was quarterback Aaron Rodgers’ tipped-ball interception on the opening series that gave life to a Colts team that had looked emotionally drained in the first half. There were six consecutive scoreless possessions where the Packers managed five total first downs. There was the defensive meltdown that allowed rookie quarterback Andrew Luck and the Colts to put up 27 second-half points. There were the two missed field goals by Mason Crosby.

But if you really want to fret about a Packers team burdened with an unthinkable 2-3 record, then forget all about the second half. You see, it was the first half that showed definitively how far removed the Packers are from the team that steamrolled through the 2011 season with a 15-1 record.

Sure, the Packers took a seemingly insurmountable 21-3 halftime lead over the Colts. But it easily could have been 31-3 had the Packers taken advantage of every opportunity that came their way. And last year, that score might well have been 41-3 because the Packers’ aggressive, go-for-the-throat offense would have buried the Colts before they even hit the locker room at halftime.

“We’re not the same team as last year, obviously,” guard Josh Sitton said. “And we’ve got to figure out a way to win these games. We know it’s not easy to win in this league. That’s pretty obvious. When you’re up a few scores, we’ve got to close out the game.”

For whatever reason, the Packers not only can’t close out games this season, they’ve lost their killer instinct.

It could be personnel. Wide receiver Greg Jennings, a bona fide home run hitter, has been in and out of the lineup with a groin injury and didn’t play Sunday. Halfback Cedric Benson, whose running has energized the offense, and tight end Jermichael Finley missed all or most of the second half with injuries, leaving the offense seriously short-handed.

It could be that opponents are playing the Packers differently and Green Bay hasn’t yet adjusted. With Jennings sidelined or playing from the slot, defenses are keeping their safeties deep, playing aggressively on wide receiver Jordy Nelson and preventing the Packers from hitting the deep passes that turned so many of last year’s games into routs.

Or it could be the Packers themselves. They simply haven’t had the laser-like focus and silky smooth rhythm on offense that characterized last season.

Sunday’s loss was Exhibit A. The Packers had the Colts on the ropes and wouldn’t — or couldn’t — take them out.

“Last year, we knocked teams out offensively; we didn’t defensively,” cornerback Charles Woodson said. “We were up (Sunday). Forget about what the offense did. We didn’t keep them out of the end zone and they went down and scored points, scored touchdowns. That’s on our heads.”

Certainly, no one will argue Woodson’s point that the Packers defense needs to improve. But this is first and foremost an offensive team. Coach Mike McCarthy’s offense has been so good it has masked defensive deficiencies for years.

Sunday, that wasn’t the case. Although the Packers scored 20 first-half points, they left many more out on the field.

On the opening series, Nelson got behind the defense and would have had a 67-yard touchdown had not Rodgers overthrown him. A dropped pass by wide receiver Donald Driver stymied one drive and consecutive drops by Nelson and Finley short-circuited a promising 2-minute drive. Even the defense chipped in. Linebacker Nick Perry forced a Luck fumble, but he was correctly penalized for using the crown of his helmet and the Colts retained possession.

With the exception of Finley dropping passes, none of those things occurred with any regularity last year.

“We’re not playing with clarity,” McCarthy said. “We’re not taking advantage of clean plays. When you have a clean play, you expect execution and productivity and we didn’t get it done.”

It was the largest halftime lead the Packers have blown since 1957, when they held a 24-3 advantage over the Los Angeles Rams before falling 31-27. To explain it, some will point to factors such as injuries, abandoning the run, bad calls by the officials and the great emotion the Colts played with for their hospitalized coach in the second half.

But a routine performance by the Packers offense would have taken every one of those factors out of play by halftime. Just like it did last season.