Matt Stajan had two goals and David Jones also scored for Calgary.
"We just got outworked. It's the same story," Flames wing Lee Stempniak said. "We had a bad first period. We didn't play smart, we didn't play hard. That was a talented team and they took advantage of the odd-man rushes."
Leading 1-0 already, Dallas scored four straight goals to open the second period. Seguin began the outburst at 1:19 by knocking in his own rebound after Benn got the puck to the front of the net.
The same combination struck again at 4:25, and again 4 minutes after that. That goal, which made it 4-0, ended the night for goalie Reto Berra, who was replaced by Karri Ramo.
"We need to get back to our game and get back to our game quick. That's unacceptable," Flames coach Bob Hartley said. "We're basically giving games away with unnecessary mistakes. Basic, simple mistakes that shouldn't happen at this level."
Seguin scored 30 seconds into the third period to extend the lead to 6-1. After two Flames goals, Seguin got his team-leading 12th of the season on a power play at 16:21 for the first four-goal game of his career.
When the Flames and Stars met three weeks ago, Dallas was last in the West, four points behind the Flames. Since that night when Dallas won 5-1, Calgary has gone 2-7-1 while the Stars have gone 7-2-2.
"It all falls into the same thing — not being mentally sharp, making wrong decisions, having turnovers, losing your guys, losing battles," Flames defenseman Dennis Wideman said. "The frustration has been building for the last four games now and it kind of accumulated tonight."
NOTES: Calgary has given up the first goal in seven of its last eight games and has been outscored 11-1 in the first period. ... Nichushkin, Trevor Daley and Goligoski led the Stars at plus-4. ... Wideman played in his 600th NHL game while RW Tim Jackman appeared in his 400th for the Flames. ... Flames D Derek Smith was scratched. ... Mike Cammalleri's three-game goal-scoring streak ended for Calgary.