I wouldn't even play desecration demon in rakdos aggro. It is a nice card, don't get me wrong, but it doesn't do the kind of jobs that Exava does. You can't pressure them forever and though you do have some anti-wrath tech (exava is also a human for necromancer), exava doesn't let the opponent get to try to board wipe after you drop it. Instead, it gets to swing first. On the draw, exava is also more dangerous than DD. Say they verdict you after your third turn. Exava turn 4 and swing is a lot more threatening than DD as they're going to have to deal with it right away. Then comes whatever else to follow which probably also has haste. You're getting in damage right away and forcing them to deal with you rather than giving them time to get ahead and not have to deal with the problem till a turn later when a creature lacking haste could attack. This is why creatures like rakdos shred-freak make the cut.