I would contact the company and let them know your feelings. I would tell them you want to be honest with your readers and leave it up to them whether or not to publish a harsh review. If I absolutely hated something, I probably wouldn't give it away to my readers, but that's just me.
So sorry about your experience. I've been lucky that I haven't had something yet that I couldn't stand but I'm sure it's bound to happen one of these days!
I would do as Kristen said and contact the company. I would let them know your feelings, and ask if they'd rather you just post the giveaway without the review (if you feel like you would still want to), post the review as you truly feel it should be, or if they'd rather you step away completely.
Regardless of which option you choose, by contacting the company you are at least giving them valuable feedback, which I am sure they will appreciate.
I am brand new to blogging and have not done a giveaway yet but have made a form for all companies to fill out before agreeing to sending me a product. On one of the MANDATORY checkboxes for MY terms say "I understand and agree that if ecomodern mom is not satisfied with the product a review will not be written and used or opened products will NOT be returned". In the future you could consider doing something similiar, however I am a little concerned if that'll scare companies away. For the current situation I would do as already advised and contact the company and be honest with them and ask if they still want a review even if it's a negative one or offer just the giveaway. In the long haul you are doing them a favor but giving them HONEST feedback. Good luck!
I agree about NOT doing it. A lot of people believe in honest reviewing and I agree with that except I don't believe bashing a company or it's product is in good faith of the reviewing process. I'm sure there are others who liked it and can review it. Feed back is the most important part and you should definitely let them know. I would also shy away from the giveaway.
I've done two product reviews that I really didn't like. One was for some hard candies that were pretty nasty, and I simply stated in my review why I didn't like them. But I was sure to include other things about that product that people might, in fact, like that I didn't. The other was for a line of beauty products and I thought they were horrible. I deliberated on what to do and decided that if I could find one pleasant thing to say about the company, then I would go ahead and post the review and just say that the products didn't work out well for me and why. So I did, and in both cases I didn't burn any bridges with the PR companies or feel badly, because I did give my honest review. And, quite frankly, I think it's refreshing to see a review that isn't glowing every now and then on other blogs...at least you know there is honesty out there and not just rehashing of the fact sheet.
In your case, could you post the review but opt out of the giveaway?