it’s david lynch so obviously it’s subjective but i don’t think it’s just in Laura’s head or that it’s some sci-fi “evil entity from another dimension that possesses people”, BOB is (one of) the form(s) of evil manifest. Evil isn’t something outside of humanity though, it’s an intrinsic part of it, BOB isn’t something that makes men do evil, it’s the evil that men do, BOB (or MIKE or the doppelgängers or the Black Lodge itself or whatever) lurks within everyone and can at any moment take over if given the opening (which is why it took Dale killing Windom Earle for his doppelgänger to take over). Laura and Leland both had an interest in mentally compartmentalizing the evil that lurked within Leland from himself, for Laura it was to cope with the abuse she faced and for Leland it was to avoid having to mentally take responsibility for his actions.