Having robbed Alex Gilbey as he tried to collect a pass from Jack Tucker, Williams fired past keeper Filip Marschall from the edge of the penalty area.
Worse followed for MK Dons on the hour mark - again after an error at the back.
Crawley closed down and won possession once more, and after Dan Harvie misjudged the bounce of the ball, Orsi nipped in front of him and finished emphatically.
Williamson’s side pulled one back in first-half stoppage time. Jack Payne’s free-kick from the left was nudged back across goal by Tucker to Dean, who had a simple close-range finish.
The goal was Dean’s 19th of the season and he was arguably fortunate to still be on the pitch, having earlier appeared to stamp on Williams.
Three minutes into the second half, Crawley’s five-goal cushion was restored.
Again they scored after stealing possession. Liam Kelly seized the ball in midfield and continued into the box where he collected a return pass from Klaidi Lolos, drew Marschall from his line and, rather than shoot, opted to tee up Orsi for a tap-in.
When Kelly handled while blocking Ellis Harrison’s cross, the resulting penalty gave Dean a chance to reduce the deficit again and reach the 20-goal mark, but Addai dived to his right to save the spot-kick.
Crawley were relentless and Roles had a long-range strike pushed against the post by Marschall before scoring late on.
After evading two challenges to set up Orsi, Roles pounced on the loose ball after his team-mate was tackled and thumped it into the bottom corner.
And in stoppage time, Ronan Darcy found space on the left, collected Nick Tsaroulla’s pass and crossed for Orsi to complete his hat-trick.
