What, you thought this would be easy? Well, it seems the Islanders did too.

Is there another reason for their inexcusable mess of a performance on home ice Tuesday night? For why, in the late stages of a playoff race in which every game is a must-win, Mathew Barzal — benched earlier this season over a failure to back-check — stopped skating to watch as Tyler Bertuzzi and Frank Nazar went up the ice on a two-on-none? For the way the Islanders came undone from there?

Surely not.

The game ended 4-3 to the Blackhawks, whose urgency and intensity outstripped that of the Islanders until the last period, when a frantic attempt at a comeback came up a hair short. If the Isles’ loss in Ottawa was a bad night, and their loss in Montreal felt like a shock to the system, then this was something far worse.

This one should make them angry, and it should make their fans angry. If the Islanders go on to miss the playoffs — and they are below the cut-line again after the results rolled in Tuesday — this one will stick out like a sore thumb.

“We had some turnovers that ended up in our net and then you get a shot that goes off a body behind the goalie and it’s 3-1. Other than mistakes, it has nothing to do with our intensity or will to win,” captain Anders Lee said, understandably shielding his teammates. “We just messed up. That’s gonna happen. You saw that will the rest of the way tonight.”

Indeed, it was there over the last 20 minutes. Before that, though, the Islanders defended casually, and let momentum spiral against them.

After taking a lead inside a minute and dominating the first 10, Barzal turned the puck over and then failed to back-check, allowing Nick Lardis to follow a two-on-none rush and tie the game after Bertuzzi’s initial shot missed the net.

“I feel like we play as a team, we win as a team,” coach Patrick Roy said, later adding he didn’t consider benching Barzal. “There’s mistakes in the game. It’s part of the game. The back-checking, for us, is important.”

Making matters even worse, Tony DeAngelo was hurt while back-checking on the same play, leaving the Islanders with five defenseman and without their top two right-handed defensemen, as Ryan Pulock missed a second straight game with a lower-body injury.

DeAngelo did not return, with the club citing a lower-body injury and Roy saying he would be evaluated Wednesday.

It took the Islanders until the third period, when goals from Simon Holmstrom and Cal Ritchie, along with an urgency level that finally looked appropriate, brought them within one and made for a close ending.

Indeed, the Islanders so nearly got away with it, getting a six-on-four power play with 27 seconds to go and coming within a starring, 44-save Arvid Soderblom performance of tying the game.

Not one bit, however, does that excuse everything that came before.

The Blackhawks had scored two more times before the end of the first, chasing David Rittich, then added a fourth goal against Ilya Sorokin 6:02 into the second.

The missed icing call that preceded the eventual game-winning-goal from Nazar was as bad as it gets — so bad that Roy said the officials admitted to him they’d made a mistake — but the Islanders also needed to play to the whistle that never came.

With the noted exception of Matthew Schaefer, who recorded his 30th assist on Lee’s goal 49 seconds into the match and had a ridiculous 31:59 of ice, the defense corps served up a dog’s breakfast during the consequential stretch that spanned from 10 minutes into the first through the end of the second.

\Carson Soucy was on the ice for three goals against, Scott Mayfield for two while Adam Pelech committed a pair of penalties.

Adam Boqvist, for good measure, allowed Bertuzzi to beat him to a loose puck in the crease to make it 3-1 after Rittich — who stopped nine of 12 shots before being pulled after just one period — could not secure Alex Vlasic’s point shot.

“It’s fine that we were making mistakes, but I feel like we should have recovered from it,” Roy said. “We should have back-checked and tried to protect the netfront on that first goal. … I thought we were playing a really solid game, controlling the game, playing really well. A couple turnovers made the difference.”

This is a playoff race in which no one seems to be beating themselves. If the Islanders end up being first to do so, they will be at the bottom of the pile, and they will deserve it.