Since henna is not alkaline, the developer won't likely do much to lightnen your hair.

Chemical permanent hair colours work by opening your hair shaft (alkalinity, usually ammonia does this), and lightening the natural pigments (note: this does not work on artificial pigments!) and then depositing and developing the artificial pigments into the hair shaft.

Peroxide (developer) does not much lightening on its own. The alkalinity allows it to do its work within the hair..

Unfortunately f i you really want to see henna results, you'll need to lighten with peroxide and some form of alkalinity.. either permanent colour or bleach.

Henna contains lawsone, a pigment, which bonds with keratin (nail and hair protein). It will not further damage your hair and will be gorgeous over lightened hair.

Good luck!