They ended the Super League leaders’ five-match winning run with his drop goal in the final second of Thursday night’s thrilling game at Wheldon Road.
The 25-24 success was the first time Castleford had beaten Saints in more than seven years, but former Kiwi star Roberts was nervous before his crucial play.
He had knocked on in front of his own posts when the game was level with just 90 seconds remaining.
“Myself and the boys had a joke in the sheds as I felt like I’d gone from villain to hero,” the 29-year-old said afterwards.
“I was disappointed as it was a bad error and pretty crucial, but I was fortunate enough to make up for the mistake straight after and that made me feel better after.
“I was pretty fortunate to get the drop goal and credit to the boys, they hung on in there.
“We’re getting good combinations going, something we’ve been working on the last few weeks, and if we keep improving we’ll go well at the back end of the year.”
It was Castleford’s fourth straight win and came after beating previous leaders Leeds Rhinos the week before.
It was also Roberts’s first successful drop goal since 2007 but the player, who joined from Melbourne Storm over the winter, admitted: “I can’t remember my last to be honest. I was just happy to have got it.
“I did know it was the last play of the game so I thought I might as well have a crack at it and I got it.”
The Samoan international signed as a stand-off, but has been used effectively as a No 1 recently due to Luke Dorn’s injury. Roberts, who also set up tries for Justin Carney and Jake Webster, said: “It’s definitely new to me but I’m working on the position and getting more confident every week I’m there.
“If I can keep improving and doing a job with the team then I’m happy with that.
“The fans were great again. It was probably the best night I’ve experienced here at Cas. I definitely think they played a big part in us finishing so strong and that’s why we hung in there – for them.”
Castleford now have a fortnight without a game.
They next host Widnes Vikings on Sunday, July 5, due to the Challenge Cup, but are well in contention for a top-four spot.