Heather's situation basically, she was raised by a family who really doesn't seem to care for her, her boyfriend and best friend have sex behind her back, the townspeople decide to lynch her for simply being related to the Sawyers, and she didn't even know until the night they found out. No wonder she sides with Leatherface at the end.
Although, considering Heather doesn't seem to really care that Leatherface killed all her friends and mutilated their corpses, and let him murder a police officer who was only doing his job, maybe they were on to something.
Granted, no one was too sad to see Nikki and Ryan go, but Kenny...
During the opening scene after the Sawyers house is burned, Heather's biological mother survives but critically inujred to the point where she can only hold her baby. Gavin, one the the mob members and Heather's soon to be adopted (and abusive) father walks up to her and all she can say is "Help us" and after Gavin takes the baby, he kicks Heather's mother in the head, killing her.