I do think it was hasty, but I also find it completely within the nature of the character. Prior to that point, we're both told and shown that Susan is an upfront, cheerful, realistic girl. Knowing that, her decision to go to the house is completely within character: upfront, because she faces her problems head on (remember her confrontations with her mother over both Ben and Floyd. Neither could have been easy, but she bit the bullet and did what she had to do. And she decided to move out not from pique, as her mother suggested, but because she decided in a clear headed manner that it was time). It was both cheerful and realistic because she is not one to fear shadows, not even after seeing Mike rise. She's even aware that what she is doing is classic horror movie, "What the hell is that dumb ***** doing?" material, but she can't change her basic nature, which is to be optimistic and realistic.