After Romeo arrives at the tomb, he instructs Balthasar to leave him as he enters "this bed of death" and to not attempt to re-enter. Romeo then encounters Paris and slays him; this act furthers Romeo's highly emotional state in which he delivers his soliloquy. Throughout this speech, Romeo employs the stylistic device of apostrophe.
Apostrophe is a literary device in which a speaker directly addresses an absent person or a personified quality, object, or idea.
In Act V, Scene 3, Romeo addresses his lips:
O youThe doors of breath, seal with a righteous kissA dateless bargain to engrossing death. (ll122-124)
He also addresses Tybalt, to whom he says he will pay his debt by killing himself. He speaks to death, and the apothecary: "O true apothecary" (l130).
This device is often used in poetry and speeches to add emotional intensity, and certainly Romeo's speech is highly charged with emotion. In addition, it is typical of Romeo to use apostrophe here since he has done so previously on at least two occasions in which he has also been in an emotional state. (e.g. “Arise, fair sun, and kill the envious moon." 2.2.5 and "Then I defy you stars!" 5.1.24). Moreover, since Romeo met and fell in love with Juliet, he has been continually in an emotionally charged state, so the use of apostrophe is certainly in character.