The tiger in Blake's "The Tyger" is presented in such a way that none of the choices you offer--negative, positive, or neutral--really fits.
"The Tyger," from Songs of Experience, and its companion poem, "The Lamb," from Songs of Innocence, present opposite sides of the same existence, opposite sides of the same creator, and opposite perspectives of the same existence. Thus, the tiger is not negative, not positive, nor neutral--it just is.
Blake did not believe in dichotomies; he did not believe in good and evil in the traditional sense. The world is not simplistic and neither are people. Existence cannot be labeled or categorized. Human beings are not completely good, or completely evil, and neither are the creator or nature.
The lamb represents innocence and an innocent perspective. The tiger represents experience and an experienced perspective. Neither is good or bad, or negative or positive--the lamb and the tiger just are. That's just the way it is; that's existence.
Furthermore, the same creator that made the lamb also made the tiger. And if he can make two creatures so opposite, then the opposites that exist in the creatures, must exist in the creator, as well.
Look at the reverence and awe revealed in the stanza that opens and closes the poem:
Tyger, Tyger, burning bright,
In the forests of the night;
What immortal hand or eye,
Could frame thy fearful symmetry?
The tiger burns bright in the forests at night, and it takes some kind of immortal to create this beast. But there's nothing evil here. A tiger isn't evil. A tiger is a predator that kills to eat. Tigers don't kill for fun, people do that. The tiger is presented with reverence and awe.
Also, the "Did he who made the Lamb make thee?" line is a rhetorical question. The line is the conclusion to everything in the poem that's come before it, and makes the point that yes, the same creator that made this fearful beast also made the lamb. That is the point.
Just to be practical, if your assignment requires you to choose between the three choices you mention, I'd suggest using positive, even though, again, the tiger is just a fact of life, or of existence. But positive is certainly more accurate than either of the other two choices.