Snyde vs. Phelps case. A few points about this topic:
This legal case is a landmark Supreme Court case about the right to free speech; it is one specific instance of the much broader debate about what “free speech” means in this country.
Debate about free speech and what kind of speech should be protected and what type of speech shouldn’t be protected have been raging in this country since it first became a country . . . and there is no sign those debates are ever going to go away.
Debate about what kind of speech should be tolerated and what kind of speech should be banned are probably more intense right now than at any time in the past.
The concept of “free speech” has never been and never will be absolute. All societies put restrictions on some kinds of speech — slander, libel, physical threats against other people, lying in legal cases and on legal contracts, etc. etc. — and they always will. The debate is always over where exactly to draw the line on provocative or offensive or potentially harmful speech that may or may not rise to the level of being banned.
By framing this essay topic around a specific legal case, it will allow you to focus your essay just on this one case in order to make your essay more manageable, or you may expand your to include arguments about banning or not banning hate speech in general.
Free speech debates are somewhat, though not entirely, philosophical arguments that are not necessarily dependent on a lot of outside research. It might be possible to write this essay without having to do a lot of outside research, though some research will certainly be helpful, and depending on your argument may actually be necessary.