"And it die" — the Torah describes what happens when a deposited animal dies in the guardian's care. The Mekhilta specifies: "at the hands of Heaven." This means natural death — the animal died of illness, old age, or some other cause beyond human control.
"Or be broken" — this refers to injury by another animal. "Or be seized" — this refers to seizure by robbers. In all three cases — death, breaking, and seizure — the unpaid guardian is exempt from liability, provided he can swear that the loss was beyond his control.
But the Mekhilta refines this. For death, the exemption is clear: natural death is always beyond the guardian's power to prevent. But for breaking and seizure, there is a question. What if the guardian could have rescued the animal? Perhaps the attacking beast was small enough to fight off, or the robbers few enough to resist.
Rabbi Eliezer resolved this by using death as the benchmark. Just as death is characterized by the impossibility of rescue — you cannot prevent a natural death — the other categories also require impossibility of rescue. The guardian is exempt for breaking or seizure only when he genuinely could not have saved the animal. If rescue was possible and he failed to act, the exemption does not apply. Helplessness, not mere difficulty, is the standard.