Depression grows stronger when people are alone "I'm the poison in your bones" -> Depression tends to have a paralysing effect, it prevents people from doing anything, even physically "My love is your disease" -> Depression tends to be "comforting", it makes people think that being depressed is what should be, and won't want to change it "I won't let it set you free" -> Same as before, depressed people think that they don't deserve to be "cured", because it is what depressive thoughts tell them "I'm underneath your skin" -> Depression will also hide itself from the outside. Depressed people will try to look happy, because they don't feel like they "deserve" to be really depressed. So while they act as usual, they feel depression creeping under their looks, unseen but far from powerless.">
This song can talk about a lot of mental issues, that tend to hide themselves. The song being focused on the person, and not on "victims", I'm not sure about the sadist interpretation. It doesn't feel like someone trying to slowly manipulate and hurt someone else, but instead of a part of the mind slowly taking control. If you link it with something like depression, it seems to fit relatively well.
Depression in its chronic form usually comes slowly, in a way that makes it hard if not impossible to detect. It takes its place in the "dark corners" of the mind, insecurities, and such, which fit "the cobwebs and the lies".
While depression can be seen as an ennemy to fight, it usually ends up in self-harm, people will hurt themselves hoping that it will reduce the depressive thoughts, even momentarily. Thus the "you would never see me cry till I make you".
A lot of other verses can also follow this logic. "When you think you're all alone" -> Depression grows stronger when people are alone "I'm the poison in your bones" -> Depression tends to have a paralysing effect, it prevents people from doing anything, even physically "My love is your disease" -> Depression tends to be "comforting", it makes people think that being depressed is what should be, and won't want to change it "I won't let it set you free" -> Same as before, depressed people think that they don't deserve to be "cured", because it is what depressive thoughts tell them "I'm underneath your skin" -> Depression will also hide itself from the outside. Depressed people will try to look happy, because they don't feel like they "deserve" to be really depressed. So while they act as usual, they feel depression creeping under their looks, unseen but far from powerless.