West Coast have given up eight straight goals to turn a 33-point lead into a 20-point loss to the Saints.
The Eagles stopped to a walk in the last quarter as the Saints ran rampant to record a 15.12.102 to 13.4.82 victory.
Jack Petrucelle booted four goals and was West Coast's best in front of goal while Jack Darling, Josh Kennedy and Oscar Allen kicked two majors.
Coach Adam Simpson left to lament missed opportunities.
“We thought St Kilda’s pressure was there all night,” Simpson said.
“It elevated in the second half and we couldn’t handle it.”
Veteran Shannon Hurn suffered a calf injury during the early stages of the match and Simpson he would be sent for scans.
“He’s done a little calf. A little soleus,” Simpson said.
“We’ll get it scanned. Soleus’ aren’t terrible but it probably rules you out for a week or two.”