I'm going to continue to negotiate an offer after accepting an offer from another company
Are you attempting to negotiate the salary?
Yes it is. I want to negotiate a higher salary compared to the other offer.
It's unethical to negotiate with one company if you already accepted another job offer. There is no point in keeping both companies in uncertain about your joining.
I'm going to do it anyways.
It's important for you to hold on to the company that you have already accepted a job offer for. Try to remember that salary is not the only aspect of a job. You might find that the company environment and culture of the first offer is more acceptable for you.