grammaticality - why is present tense used with before instead of past tense in this sentence?

The sentence you offer lacks tense parallelism, and most people would consider it to be grammatically problematic.

There are time when there is a tense shift to signify a contrast between two reported events. Often the earlier activities are described in the past perfect and the later activities in the past. That is not want is occurring here.

All of the events occurred sometime in the past, and while there is a sequence, using the present and shifting to the past is a bit jarring. Better approaches would be to narrate all of the activities in the present or all in the past:

And then just before the killer gets the girl and stabs her with this giant knife, the power goes out and the TV goes off.

or

And then just before the killer got the girl and stabbed her with this giant knife, the power went out and the TV went off.

The first example uses the present to narrate events that have occurred in the past, but that is a standard convention, creating a scenario as if the events are currently being viewed.

The phrasing makes clear what the sequence is. If you felt an overwhelming need to use tenses to emphasize the sequence, you could say (but it would sound a bit stilted):

And then, just before the killer got the girl and stabbed her with this giant knife, the power had gone out and the TV had gone off.