What is so “wicked” about playing records backwards?

Good and Evil are subject to the opinion of the majority.
Anything that moves against the current will be perceived as evil, as sinister.
The Sinister or Left Hand path is a movement against the tide of automatic organic life.

If the mechanical organic human creation is the equivalent of the Shakti current, then we would say it is positively charged. The otherworldly immortal presence would then be the Shiva current and would be negatively charged.

Positive and negative describe the way the electrical current flows from one polarity to the other, whether it is coming or going, attracting or repelling.
The use of these two terms must not be confused with morality of any kind.
Positive is not in this case a synonym for good, negative is not a synonym for evil.

You could say then that the Sinister way involves reversing the flow of electricity so that Shakti adores Shiva rather than letting him rain down upon her.