Michael Amadio extended the lead to 3-1 at 8:24, and Marchessault made it 4-1 at 9:04 when he redirected Janmark's centering pass.
"Urgency. Energy. Emotion. Those things," Sutter said. "One team was way more focused and urgent."
Roy gave the Golden Knights a 5-1 lead at 17:36, tipping in Shea Theodore's point shot by Vladar for a power-play goal, and Karlsson scored at 18:19 of the third period for the 6-1 final.
"We came and we did what we needed to do," Vegas coach Pete DeBoer said. "We're obviously a more desperate team than they are as far as the standings go. I thought we came in, the first five minutes we were feeling it out, but the first goal was big for us to tie it up. I really liked our game. We had to win that game. They're in a position where they don't have to win this time of year."
NOTES: Thompson, who was making his first start since April 1, is 5-0-0 with a 1.59 goals-against average and .949 save percentage in his past five starts. ... Karlsson's goal gave him 300 points (125 goals, 175 assists) in 526 NHL games. He has scored nine points (three goals, six assists) in his past eight games. ...Calgary has scored first in an NHL-leading 48 games. ... Vegas is 31-2-0 when leading after two periods. … Eichel has scored seven points (five goals, two assists) during a six-game point streak. … Marchessault has scored 15 points (four goals, 11 assists) in his past nine games. … Flames defenseman Erik Gudbranson was minus-4 and had one shot on goal in 17:03 of ice time after missing three games with an undisclosed injury. … Calgary defenseman Oliver Kylington had two shots on goal and one blocked shot in 18:17 of ice time after missing six games with an upper-body injury.