Following that principal in Magic, Deathrite Shaman’s ability states that, “Hey, pay {B} and tap me, then I’ll get to eat an instant/sorcery card from a graveyard and shoot your opponent for 2!”, so once you pay its costs (aka, everything LEFT of the colon), the ability (Everything RIGHT of the colon), goes onto the stack.
Alas, going on the stack means your opponent can respond to it. And if they cast Mr. Snappy, giving that particular instant card flashback, and they then cast that card, it’ll resolve and be exiled due to flashback; we’re now left with Deathrite’s ability on the stack, targeting something that’s not there.
Since the ability has no legal target, it will be countered upon resolution. Deathrite won’t be able to hit your opponent for 2 life.
P.S. It may be more strategically sound to target a sorcery card instead, since they will not be able to cast it once it gains flashback due to the fact that your own Deathrite’s ability is still on the stack.
Today’s Rules Tip written by Eddie Cheung