Why does Paris think Romeo has come to the tomb?
Paris thinks Romeo has come to the Capulet's family crypt in order to vandalize it or to desecrate the dead bodies of the Capulets (namely, Juliet and Tybalt) that lay within. This is perhaps an odd or surprising sentiment for a reader to encounter, but we must contextualize it by recalling that Romeo and Juliet's love affair and marriage have been keep a secret from most of the major players in this narrative; Paris does not know that Juliet--the woman to which he is betrothed--has actually already been wed to another man. We can see this confusion in the small speech Paris makes when he spots Romeo approaching the tomb:
This is that banish'd haughty Montague,
That murder'd my love's cousin, with which grief,
It is supposed, the fair creature died;
And here is come to do some villainous shame
To the dead bodies: I will apprehend him.
Paris wants this "condemned villain" to suffer the vengeance that he deserves: that is to say, death. In his state of grief, that of "a desperate man," Romeo does not even attempt to correct Paris or justify his actions. Instead, they duel, and Paris is slain.