A knee injury to star defender Shannon Hurn was the only lowlight for the Eagles in the 18.15 (123) to 4.6 (30) thrashing - their second straight win to start the year.
Luke Shuey and Nic Naitanui starred in the midfield and Josh Kennedy kicked four goals in his 100th game.
TEAM SONG: Watch the boys celebrate Kennedy's milestone
It was the biggest loss Roos has endured as a senior coach, beating the previous-worst margin of 73 when he was at the Swans.
By contrast, the Eagles were ruthless a week after crushing the Western Bulldogs by 65 points to open their season.
Hurn was subbed out in the second term after suffering the injury in the first few minutes of the match.
The Eagles took control from the start and kicked four goals in 13 minutes, putting the Demons out of the contest.
West Coast bullied Melbourne at the clearances, winning them 13-3 in the first quarter.
When the Eagles weren't surging forward, Melbourne was compounding its own misery by turning the ball over.
Five of the first eight West Coast goals were direct results of the Demons coughing up possession.
Melbourne kicked one behind in the first term as West Coast took a 40-point lead into the first break.
The Demons did not kick their first goal until seven minutes into the second term, through Shannon Byrnes.
He also kicked Melbourne's second goal at 23 minutes into the third.
PHOTO GALLERY: All the action
While it was a poor day for most Melbourne players, co-captain Nathan Jones again stood out with a gutsy performance in the midfield.
Jones, who has won the last two club best-and-fairest awards, was relentless and finished with a game-high 34 possessions.
Eagles coach Adam Simpson wasn't getting too carried away with his side's unbeaten start to the season.
"We're happy. Don't get me wrong. We're pleased with the result and where we are. But it's a long year and we haven't really been thrown in the deep end and tested yet," he said.
"I'm sure that's coming."
POST-MATCH: Adam Simpson reviews his team's 93-point victory
Roos, who has taken a patient approach during his brief time at Melbourne so far, was left frustrated after the defeat.
"Some of the stuff today … guys missing chest marks. I haven't seen that before, so it's an eye-opener for me. Some of the errors were just incredible," he said.
"If you get hold of the ball and you fumble it all the time, if you can't make tackles, they're the most basic things in footy. Today we were just really poor at it."
MELBOURNE 0.1 1.3 2.4 4.6 (30)
WEST COAST 6.5 10.7 14.10 18.15 (123)
GOALS
Melbourne: Byrnes 2, Kennedy-Harris, Toumpas
West Coast: Kennedy 4, Darling 3, Masten 2, LeCras 2, Shuey 2, Naitanui, Mackenzie, Cripps, Sinclair
BEST
Melbourne: Cross, Tyson, Jones, Byrnes
West Coast: Shuey, Gaff, Masten, Kennedy, Selwood, Priddis, Naitanui, Darling
INJURIES
Melbourne: Michie (cut eye)
West Coast: Hurn (knee)
SUBSTITUTES
Melbourne: Jay Kennedy-Harris replaced Jack Fitzpatrick during the third quarter
West Coast: Brad Sheppard replaced Shannon Hurn (knee) during the second quarter
Reports: Nil
Umpires: Donlon, Hay, McInerney
Official crowd: 22,226 at the MCG
