Backes had been coming to the aid of defenseman Kevin Shattenkirk, who'd been cut by a high stick.
"It shows what kind of leader he is, if you're going to go after an 18-year-old," Roy said. "Gutless in my opinion. I mean, Shattenkirk hurt himself with his own stick.
"His own stick hit him in the nose and he was bleeding and he made a pretty big act."
The Blues didn't quite take the bait, preferring to forget about this one and get ready for a matinee Sunday at Chicago.
"He always has something to say after every game," coach Ken Hitchcock said of Roy. "So you guys pick up what he wants. I'm worried about my team and the way we played, OK?"
Hitchcock did, however, suggest that forward Patrick Bordeleau left his feet on the check that hurt Shattenkirk when his stick was jammed into his face. Hitchcock also mused that he probably should have given rugged Ryan Reaves more ice time after the game had gotten out of hand and added that it was more important to get his team back on track rather than call Gabriel Landeskog "gutless or whatever."
Hitchcock referred to the confrontations and likely aftermath as the "debris around the game." Backes said Roy was "entitled to his opinion."
"There were some liberties, I feel like," Backes said. "I drop my glove and never throw a punch and end up with whatever many penalty minutes out of the ordeal.
"It's frustration, everything boiling over for a lot of guys after they don't respond well."
Semyon Varlamov tied the franchise single-season record for goalie wins held by Roy and Paul Stastny had another big game in his hometown in the penalty-filled game. The Avalanche avoided a season series sweep against St. Louis after losing the first three meetings.
"Our power play was sharp, penalty kill was rock solid, goaltending was phenomenal again," Roy said. "I just felt like we dominated them."
Stastny had a goal and two assists and MacKinnon had a goal and an assist for Colorado, which has won six in a row. Roy became the fifth coach in NHL history to win 50 games in his first season.
"It was kind of a big game for us," Stastny said. "It got chippy at the end but we stood up to that and will deal with that."
Varlamov made 31 saves for his second shutout of the season and NHL-leading 40th win, matching Roy's total in 2000-01.
The Blues fell two points behind Boston, which beat the Flyers 5-2, for first place overall. They've been dependent on goalie Ryan Miller with Vladimir Tarasenko out since mid-March with a hand injury and Alexander Steen also sidelined, and scored two or fewer goals for the seventh time in 10 games.
There were several scrums and frustration boiled over for the Blues late in the third. Ryan O'Reilly scored his 27th goal on a power play with a two-man advantage after St. Louis captain David Backes and Barret Jackman both received 10-minute misconducts.
The Blues had 91 minutes in penalties in the third and finished the game with seven players on the bench.
St. Louis has been shut out on home ice for the first two times all season in the past three games, counting a 1-0 shootout win over Philadelphia on Tuesday.
Stastny, who grew up in St. Louis, has 30 points in 24 career games against the Blues. He has two goals and six assists in a seven-game point streak.
The first five games in Colorado's win streak were decided by one goal, two in overtime and two by shootouts. The first three goals against St. Louis came on rebounds.
Stastny scored his 22nd goal on a power play at 1:14 of the second and MacKinnon had a wide open net at 6:20, a little more than a minute after Blues coach Ken Hitchcock called timeout.
The last coach to win 50 games in his debut season since San Jose's Todd McLellan won 53 games in 2008-09. Tom Johnson won 57 games with Boston in 1970-71, Mike Keenan won 53 with Philadelphia in 1984-85 and Pat Burns won 53 in 1988-89. ... Steen (lower body) did not dress for the second straight game and Hitchcock said he's also out Sunday.