"This movie was not scary."
The adjective is what normally determines the tense (as well as positive or negative), with です merely adding politeness to the sentence.
こわい(です) = it is scary
こわくない(です) = it is not scary
こわかった(です) = it was scary
こわくなかった(です) = it was not scary.
However, when you take a more polite/formal denial, like こわくありません, it loses its ability to be conjugated into a past tense. This is when です takes over that function and becomes でした.
No, that mixes two different tenses. The i-adjective in itself kind of includes an auxiliary "to be", which can -and should- be conjugated. The adjective determines the (present/past) tense for the whole sentence. です is just an addition for politeness, and is only conjugated if it's the only verb in the sentence that can do so.
- こわい（です） ＝ it is scary
- こわくない （です）＝ it is not scary
こわくなかった（です）＝ it was not scary
こわくありません = it is not scary (more polite/formal)
- こわくありませんでした = it was not scary (idem)