Harlequins boss Conor O'Shea was left seething with his team's poor showing in their 26-33 loss to the Scarlets.
Three tries from three Williams - Rhodri, Scott and Jordan - for the Welsh side gave them a famous victory over Harlequins at The Stoop, with the 2012 Premiership champions slumping to their fourth loss in six matches this season.
Mike Brown did cross twice for Harlequins as the England full-back was one of few players to cover themselves in any glory, with O'Shea refusing to use his squad's injury problems at lock and centre as an excuse.
"I'm angry and angry. I thought we were terrible. We made so many basic errors. I'm furious. We can talk about injuries and about various different things but I'm annoyed," said O'Shea.
"When you have a bloke [Jordan Williams] who dances along a touchline to score a try when he's easier to blow into touch, that shouldn't happen.
"There were some individuals out there who gave everything like Mike Brown, Nick Easter and Chris Robshaw, but it's not a game of touch. At times it looked like that with some of the tries they scored.
"We have to man up and not feel sorry for ourselves. We have to take the flak that will come our way and absolutely correctly come our way because we've earnt that. But it's not done and it's up to the players to react next weekend."
O'Shea also stated that Scarlets prop Samson Lee should have been shown a straight red card, after he stamped on England scrum-half Danny Care, rather than being sent to the bin. The Irishman also reserved praise for Brown after his brace.
"Yes it was a red card, but we were lucky to be only 10 points down in that first half. That decision wasn't the reason for winning or losing," added O'Shea.
"Mike [Brown] was absolutely exceptional and at times he was over-trying. He was doing everything he possibly could to drag this team by the scruff of its neck and get it back into the game.
"But that's Mike - the first name on any teamsheet you'd have. He's a competitor until the very last."
Scarlets head coach Simon Easterby was left delighted with his team's performance, after they ran in three exceptional tries.
"I'm delighted with the way we went about our business," said Easterby.
"It's vital that you win your home games in the Heineken Cup and if you can get points on the road then that's a big advantage.
"But this won't mean anything we don't get a result at home nest weekend. This was a big performance but we have to keep our feet on the ground."