McKellar was unable to disguise his anguish as the Waratahs suffered a fourth-straight defeat - and second in a row at Allianz Stadium, which had been the side's fortress until last week - to remain anchored in eighth spot.
Friday night's must-win game was as good as over at halftime after the Crusaders piled on five tries to one to take a 31-7 lead into the break.
"A pretty passive defensive display," McKellar told Stan Sport.
"(We) didn't want to fold around the corner, guys (were) loping on the short side, losing collisions and they took advantage of it.
"We've just got to run hard, tackle hard and show that we actually care."
McKellar's halftime spray seemed to have worked, with back-rower Langi Gleeson grabbing his second try of the game and lock Miles Amatosero storming over in the space of eight minutes to pull the Waratahs back to within 10 points.
But the comeback was fleeting, with McKellar left furious in the coaches' box when Gleeson knocked on from a restart shortly after the Waratahs had dragged themselves back into the contest.
All Blacks ace Sevu Reece wasted little time punishing the hosts for their error, running in the Crusaders' sixth try of the night to join retired Hurricanes star TJ Perenara as Super Rugby Pacific's all-time leading try-scorer.
A last-minute consolation try to winger Triston Reilly proved merely that after flanker Tom Christie had run in the Crusaders' seventh.
The Waratahs' big loss came with superstar fullback Joseph-Aukuso Suaalii sidelined with a broken jaw.
But McKellar won't be using injuries as any excuse.
In one of the few positives for Wallabies coach Joe Schmidt, veteran playmaker James O'Connor once again impressed during a 25-minute cameo off the bench for the Crusaders.
After revealing last week that O'Connor was on his radar for a call-up for Australia's hosting of the British and Irish Lions, the 34-year-old set up Christie's try and also saved one with a cover tackle on Reilly.
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