What an awful story about Margarita and very sad to hear but I do have to say the islands are as safe/unsafe as the rest of the world for those wearing excessively blingy and obviously expensive jewellery.
I live here in the caribbean and am lucky enough to have some expensive jewellery. I do not feel threatened in the places I choose to wear it at all. However, I also exercise caution in some areas I am unsure about.
I can't comment on Venezuela/Margarita as for all it's oil there is real poverty BUT for the other islands you are visiting remember these nations are home to proud people. Don't be discouraged from meeting them.
As an aside it's always nice to buy the locally made jewellery on the islands anyway as souvenirs of your fun times. 8)