I am trying to slice in half a pentagram, but it keeps adding an extra line/s... two lines actually, but both are along the same path as the division line. How do I keep it from adding the new lines?

You need to use "cut" instead. Although if you find it's creating sub paths with extra lines, then you can remove the lines using the "Delete segment between two nodes" button on the toolbar.

Ok, now neither option, cut or divide, is actually dividing the object... confused.

Maybe you should show or tell us specifically the steps you're taking?  There are several ways to "slice in half a pentagram" or any object.  And as well, there are probably a few different ways a pentagram can be illustrated.  It's very hard to help someone use inkscape, when we speak in generalities.  While on the other hand, we usually get it right the first time, when we have specifics  🙂

Especially your SVG file would be nice.  But screenshots with detailed text would still help.

@CroisEnTois you can use Path->Cut Path to get the end result instantly.

Thanks. I thought I had tried that but obviously not.