I think that the killer is (Dr.) Leslie Webber, Laura's mother.
Her reason is that on that night, she followed Laura to the attic (She was worried because Laura had be acting strange since Rick showed up for the wedding) she over heard all about what had happened that night years ago when Laura had accidently killed that Rick's mistress Theresa (I think). She was angry and upset that Rick had drugged Laura back then and upon hearing Laura scream for Rick to "stop" she rushed in the attic door and hit Rick with (whatever was handy) thereby killing him. Laura saw al this and urged Leslie to hide before she was discovered by Luke & Scott. However both Laura and Leslie where still in shock both block that scene from their minds, but Laura ended up being more traumatized by the incident than Leslie.
Leslie doesn't remember killing Rick that is why she let Laura take the blame.
Anybody else got a theory?