Williams told the club's official website: "We knew they were always going be strong from set plays. Whoever scored first would have won the game. They scored we didn't.
"We knew how they would play and we dealt with it for the most part. But you can't switch off, not even for a second, and we should have done better.
"I was picking him (Carroll) up and I should have been closer, but as a team when we analysed it on the laptop, it was a sloppy goal to give away."
Swansea are eighth in the Premier League on 34 points, having played 25 games this season.