I like the concept, but I don't like how similar they are. Elf doesn't feel like an Elf, Goblin doesn't feel like a Goblin, etc...they all seem like colorshifted variants of a single card, forced to be 8/8 and have the same downside (which really only fits in white and maybe green) for no good reason other than Because I Said So. Plus, that downside is not symmetrical--it's MUCH easier to get 8 goblins than 8 demons.
As for specifics:
Angel should have flying. Also, angel's sac ability is too powerful even though its downside is very restrictive. Consider an instant that says "exile 8 creatures." Would you cost that at WWW? Because that's how this card will be played 99% of the time. I would limit the ability to attacking creatures and probably still give it a mana cost in addition to sac. Plus, you may have intended this, but right now you have to exile exactly 8 creatures. This actually helps the power level, but makes the card even narrower and more situational, which it already is.
Demon is very weak, especially compared to angel. Again, most of the time it will not be able to attack or block. When it does, it's ability is mostly useless--deathtouch makes sense on small creatures that may not always do enough damage to kill their opponent, not on something that's usually big enough to kill anything on the battlefield anyway. Intimidate or wither, or even infect, would be better. And the sac ability is pitiful--it's usually just a kill spell, albeit one that would affect indestructible creatures, but strictly worse than "exile target creature". Remember angel's sac ability? Yeah, that was OP. This is way UP, considering that, again, 99% of the time this would be used for its sac ability and would not be able to attack or block.
Elf is actually one of the more balanced cards. Trample makes good sense on a giant monst--er,elf, there's a chance this could start attacking relatively soon since elves are common and low-cost, and the sac ability is only slightly underpowered considering this fact. I'd make it +2/+2 and again, "up to 8 creatures." Now let's address the giant monster in the room. This is not an elf. The art is not of an Elf. It doesn't have an elf's p/t. You could make it a Beast, beasts are very green and just about as common as elves, so this wouldn't make the downside too much worse.
Goblin is ok. Consider this--angel has little chance of ever using its huge body, but because of its ridiculous ability it's still OP. Goblin is the opposite--probably the easiest to actually fight with, but damn what a shitty sac ability. It's entirely possible that its ability won't kill a single creature. Actually, this would be a really cool card if it didn't have haste (that's the other thing, haste hardly makes sense on this creature because of its downside) or the sac ability, and wasn't a goblin. Just a big cheap beater that's too scared to fight unless it has a ton of goblins backing it up. If you really don't want to do this, at least omit haste (first strike might work but is pretty OP on this big a body), make the sac ability better (maybe "deals 8 damage divided as you choose between any number of target creatures or players"), and for God's sake reduce the p/t. An 8/8 goblin? You crazy.
Merfolk is my favorite, aside from the fact that it's a frikkin 8/8 merfolk. Flying also feels weird on a merfolk--islandwalk is classic for giant blue creatures but I'm not a huge fan of that either. The sac ability is very strong, but somewhat balanced considering you have to wait a turn after playing the creature to use it, and very flavorful as the classic merfolk looting mechanic. Reduce the p/t, substitute flying for islandwalk, make the downside a little less restrictive, and you've got something good.
Hope this helps...also hope it didn't come across as harsh, I just wanted to be frank. I do think this is a cool concept and right up my alley. My primary advice would be, don't be afraid to differentiate quite a bit between the cards in the cycle. it's important that they appropriately represent their respective races.