WW's are a threat to mankind -- those members of mankind, such as Andals, whom threaten their existence in the end. Jon will fight the WW's and is seen as an enemy because he has Targ blood, and the WW's recognize that and saw him as their main antagonist back in Hardhome- something most readers and viewers don't realize yet -- WW's are not really evil -- they are what they are - a wierdly unnatural manly/womanly ice dragon species that now lives in their natural environment of ice -- but the reason Winter ust magically come is that they are now in a survival migration mode seeking out new prey - the Orphans of the Greenblood - as their supply of foodstuffs and such, mabe better called life-force the Children of the Forest, and maybe some humans, is now mostly depleted - crasterbabies are a lot of things - most likely they are all born stillborn, and it is the touch of life the White Walkers give them needed to live .. I suppose that the Cave of the whitefish is where those babies are being incubated - for future generations of life -- as theat cave opposses the House of Balck and White -- and is really a Cave of Life with future generations fo Westerosi species being incubate

Natural or not, White Walkers are now within the animal kingdom -- they are a sort of ice-dragon-made man God - and longtime ago early result of Nightfort experimentation -- really just a response of sorts to what later happened at the Doom of Valyria - man becoming God.

They are really not evil or good - they are what they are, now a part of the natural environment. Maybe they win, or maybe they lose and humans survive with Aemon Steelsong being the new King -- or in the show, the borth of Tormund and Brienne's child .. whichever