By definition, one cannot “crash” a memorial service that is open to the general public without qualification. Done with good grace, attending the memorial service of an adversary or a person one dislikes or has been at odds with is a sign of respect.
Another point is that the Catholic approach to dying calls for one to forgive enemies, as Jesus Christ did when he was crucified (”Father, forgive them for they know not what they do.”) Although Catholic, John McCain seems to have missed that obligation. For an understanding and genuinely faithful Catholic, it is disconcerting that daughter Meghan McCain has taken up her father’s angry grudge against Donald Trump as if it is a treasured family heirloom.