Halliday completed 32 of 41 passes and was intercepted once. Washington State gained 464 yards of total offense in the game. Dom Williams caught three passes for 101 yards and two touchdowns.
Aaron Cantu completed 22 of 30 passes for Southern Utah but was intercepted three times.
Washington State was coming off a 10-7 win at Southern California that knocked the Trojans from the Top 25.
Cantu was intercepted at the 28 by Horton, who returned the ball untouched for 72 yards and a 28-10 WSU lead that stood at halftime.
"At that point, our margin for error was zero and we just didn't explode out in the second half," SUU coach Ed Lamb said. "We had a game that we might have had a chance to wrestle around a little bit more and get a chance to get back in the game but that pick-six made it so our margin for error was zero coming out at halftime."