This is one of the main issues of the play. Romeo is deeply hurt by and suspicious of Claudius and Gertrude's wedding, even before the ghost reveals the true manner of his death: "A little more than kin, and less than kind". One could also legitimately assume that Hamlet may also have felt cheated out of his kingship by Claudius, although that may be an anachronism.
He is also disgusted by his mother's unseemly remarriage, so soon after his father's death so that "a beast, that wants discourse of reason / Would have mourn'd longer". One fo the key issues in the play is to what extent that disgust is motivated by respect for his father and to what extent it is motivated by a sub-conscious Oedipal desire for his own mother and sexual jealousy for his rival. This, however, is a modern and overtly Freudian interpretation which Shakespeare would not have recognised.
Once he discovers the truth about his father's death, Hamlet finds justification for what he is already feeling and is able to pursue his revenge in a justifiable manner.