I am replacing a light switch that had 2 red wires(one connected to screw and one backstabbing) and the one black was backstabbing the upper part of the switch. I understand I should use the screw to attach rather than the backstab. The new switch I bought only contains screws. There is no overhead light in the room and the only thing the switch controls is the bottom half of the wall outlets. In the new switch do I need both red or can I cap one of the red ones? Or should I use a wire nut and pigtail the reds together then attach to the screw?
The simple answer is to connect your new switch in an electrically equivalent way -- pigtail the two reds together and attach it to the screw. This will guarantee the switch functions exactly as it does now.
If instead you want to disconnect one of the red wires, you need to figure out where they go. It's possible that they're actually the hot wires and the black is the switched hot, and one of the reds continues on to power other devices. In that case, you should not disconnect either of them. Or it's possible the reds are the switched-hot wires, and that one of them is no longer in use, and could be capped off, but you'll need to figure out which is which.