first off. i object completely to all these cards being able to target immortals. if there's an immortal creature on the board, that person either paid out the a** to get it there, or spent two cards to get it there. i might be fine with a single counter card like purify is to poison, but cards that counter immortality on top of doing other things, imo, are just no good.
Snatch- i think this card annoys me because it's ability is so underpowered that it begs to be abused in a way other than it's ability suggests it should be used. in fact, you designed two other cards just to help it to be abused. my solution would be to give it an ability worth using and not abusing, and then give it a real cost.
something like:
SnatchSpell
13
/ 12
Gain control of target creature.
SnatchSpell
12
/ 11
Infect target creature and gain control of it.
the cost needs to be quite high because this is in effect, worse than in instant kill card. they lose a creature, and you gain one.
consuming darkness/elemental tribute - both useless cards unless you were intending to abuse snatch.