Every time Luna goes to the park and there's a small dogs around she goes for the neck. When she get the neck she shake her head like she's trying to kill it. I know she play rough but that's Killing instinct and my girlfriend and I want to stop that.

Siberians are known to have high prey drive, some more so than others. Those that do cannot be trusted with small animals, they are food to them. I have a male that cannot be trusted with young puppies for this reason, 3-4 month old puppies he's fine with but not younger.

I would kick that behavior right now while she's still small. As it might just be play now, when she's bigger it could escalate and end in an accidental death of a smaller dog.

I think it depends on how rough she is being and the reaction of the other dog... Is she being aggressive? Is the other dog scared? Does the other dog want to still play??

When Luka play with smaller dogs they play chase, if Luka is the one chasing he will go for the back of their necks in a playful way... He does this with bigger dogs as well...

If she is actually biting on and shaking the smaller dog about then I'd agree with the above, you need to address the behaviour immediately or just avoid smaller dogs as Huskies do have a strong prey drive, some more so than others.

You should nip the shaking in the butt right away. Playing rough we see a lot, but the second Tika starts to shake, we stop her, no matter what dog shes doing it to.
All it takes is once for a small dog to be the recipient of this action. Accidents can happen as many here can attest to and you never want to put yourself in that position.
We watch ours very carefully any time they play with a small dog, and reinforce Gentle when we see them with one.

I took your Advice today, right when we enter the park she started playing with this small dog she went for the neck. I pulled her off and held her for 5min. I must of done it 3-4time before I took her home. The other dog owner kept saying its fine she's a puppy I don't think they understand me.

Good for you!
Just keep working at it, its a really important thing to reinforce. praise her too when she is playing nicely with them.
We just keep a close eye no matter what if there is a small dog at the park. If they start getting fixated we call them over to sit for a few minutes. We dont wan the prey drive to kick in to the point of fixation and potential harm.
As for other owners, like the people who say oh its ok, I dont mind if she jumps. We tell them we can appreciate that they dont mind, but its not a behaviour we want her to carry onwards, so we are trying to get it out now before she unintentionally hurts someone. They then agree. I know YOU dont mind, but our 80 year old aunt might!

Keep at it!