why do Jews use the star of David for their symbol?
is it true that the star of David can also be a symbol of the devil in goat form?
The most comprehensive answer I’ve seen can be found at Judaism 101’s entry on the topic.
I’ve never heard of it being used as a symbol of the devil in goat form. I believe you are thinking of an upside-down pentagram (with one of the points sticking straight down, so the two remaining points form the horns). A pentagram, as its name suggests, has five points, while the Star of David has six.
A Rabbi mentions that pentagrams and swastikas were also used at one time to decorate Synagogues: