What a fun question! I would recommend looking for songs about grief, especially in the face of losing someone very close to you, that could represent Romeo's perspective in this scene. As Romeo immediately goes for vengeance against Tybalt, it might be good to find lyrics that have a bit of anger to them as well as sadness.
A few possibilities I can think of include "Someone Great" by LCD Soundsystem, a song about loss or "I Die, You Die" (as covered by The Magnetic Fields), which is about grief and anger.
Another angle to take with this assignment would be to find a song that fits not with Romeo but with Mercutio's state of mind in his dying moments. As Mercutio dies, he says "a pox on both your houses," placing the blame for his own mortal wound not just on Tybalt who stabbed him but on the Montagues as well for continuing the feud between the families. If you choose to take this approach, I might recommend "Oh Very Young" by Cat Stevens, a song about someone dying too soon. The lyrics "There'll never be a better chance to change your mind / And if you want this world to see a better day / Will you carry the words of love with you" seem to reflect Mercutio's dying attitude of wishing that the feud between families would end, and that, by extension, he were leaving behind a better and more peaceful world.