I just ran this though and "I fucking hate this thing" has a -7 score, and "I fucking love this thing" is -1. "Fucking" and "hate" is negative, but "love" is positive and adds to the score. It would be improved if it could tell the difference, "fuck" itself definitely sounds negative but "fucking" can almost mean anything
It would have to somehow know that "fucking" in this case is being used as an adverb similar to "really", e.g "I really love dogs".
Another tough one would be "I don't fucking hate dogs", which actually means you like them. The sentence needs to be parsed together, not word for word :)
This does kind of start to adopt a counter-accusative tone, which could be interpreted as negative. Though it would entirely depend on context. The expletive just makes it sound angry overall.
"I don't fucking hate dogs!!" sounds like something yelled with a raised fist-finger in a dispute about dog shit on the lawn between neighbours in some lowly apartment complex.
Technically this isn't a double negative, but you are right that the negative connotation of "hate" is reversed by "don't".
"I don't have nothing" is a double negative. In "proper" English (whatever that means ;p) , the correct phrasing would be "I don't have anything" or "I have nothing".
I fucking hate this thing.
VS
I fucking love this thing.