Light switch wiring is confusing me
Oh, dear.  Who did that?

First off, the last person who worked in this box left you a mess to straighten out.  You're going to need some 12AWG THHN for pigtailing and a couple of wire nuts to fix this.


Turn the power off to this abomination at the electrical panel.
Use a pair of wire cutters to cut off the splice between the two hot (black) wires and strip the wires back enough for a good junction.
Strip both ends of the pigtail -- one black and one white is best here.
Put one pigtail with the two black wires and wire-nut them together, leaving the other end of this hot pigtail free.
Undo the tape on the white wires and free them up (this may require cutting off the existing junction if it's soldered together)
Put the other pigtail with the two white wires and wire-nut those together.
Wire the black pigtail to the AC Power L terminal, the white pigtail to the AC Power N terminal, and the red wire to the Load L terminal on your switch.
Button things back up and turn the power back on


P.S. this switch doesn't need to be grounded as it has a fixed, nonmetallic faceplate.  See NEC 404.9(B) Exception 2 for details.