That's... a bit strong. This will basically always deal at least 6, and can happily deal loads more since you can just tap your lands for no purpose.

As comparison, Magma Rift deals 5 damage for 3-and-sac-a-land.

Hmm. Power-wise, Rare certainly makes sense; but this just isn't a very appealing rare. If you look under the skin, you've just printed " - Destroy target creature". While unusually good for Red, it's just not splashy enough for rare territory.

Indeed. See Question 32 on the GDS2 Multiple-Choice Test. As Mark Rosewater put it: "no matter how big the numbers are, the ability will only kill one creature at most. Yes, -100/-100 looks impressive, but it's not much different than Doom Blade, a common card." This will end up similar to Shivan Meteor.

If you want to make the card interesting (as opposed to a red Dark Banishing that is both very powerful and has words it doesn't need), try only 1 damage per tapped land. It's good like that, especially as an instant.