Antilife Shell reads:
A shimmering barrier extends out from you in a 10-foot radius and moves with you, remaining centered on you and hedging out creatures other than undead and constructs.
So creatures outside that range can't approach you, which makes sense. But what about the creatures already inside that radius? If the casting druid is under attack by a humanoid within 5 feet of them, is that humanoid pushed back? Or, as the second parts of the spell says, would that count as "forcing them through the barrier?
If you move so that an affected creature is forced to pass through the barrier, the spell ends.
If you cast it near a creature, are they pushed away from you, or does the spell end? What about if you cast it in a field? Does the grass stop it? Would microbes in a dungeon?
It seems like the spell could be either very powerful—create a dome where there is absolutely no life—or immediately gimp itself.