City were denied a 2-2 draw at The Hawthorns when Roland Lamah's late goal was wrongly ruled out for offside, and their last four away games in the Premier League have yielded just one point.
That is not good enough, as far as Williams is concerned, and the captain has warned his team-mates against complacency with the Capital One Cup in the bag and a mid-table finish all-but assured.
"Looking at our performance as a team we could have done better," said the centre-half, whose side led Albion 1-0. "The incident at the end has gone, there's no point in talking about it.
"Some of the lads spoke to the linesman, I think he admitted it was a mistake but I'm not too sure. I've seen it on video and it's a clear goal.
"It's gone, we need to talk about our performances. When we scored, we stopped playing for some reason.
"We stopped attacking them, they're a good side and a team like that you have to finish off.
"That's the frustrating thing about this game. We did brilliantly last week against Newcastle. This time I felt we took our foot off the pedal and let them back into it.
"As a dressing room we're frustrated. As a team we have to be better than that. We've got to continue to push ourselves, it's not about being safe.
"This was a game we could have won, we let it slip. We'll be disappointed if we don't finish in the top 10."