Question

Paris attacks Romeo at the tomb because paris believes romeo

Answer 1

Paris attacks romeo at the tomb because he believes that romeo is the killer of tybalt and that he is planning to desecrate his tomb

Answer 2

2 reasons: ~Romeo was banished and should not have been there ~Paris thought Romeo was going to vandalize the Tomb which (considering he was almost a Capulet) he couldn't let happen.
He is sad about losing Juliet and wishes to spread flowers around her and promises to water those flowers every night, however he and Romeo commerce in brawl and paris dies.