Would you rather... chop off your own arm or chop off a complete strangers arm, but they come back to kill you in 30 years

Arm Yourself · Either

chop off your own arm

Modern prosthetics and reattachment surgery make arm loss far less devastating than it once was — some mechanical arms exceed natural capability.
Living with the certainty that someone will come for you in 30 years creates decades of psychological dread and ongoing guilt over harming an innocent.
You harm only yourself rather than a stranger who did nothing to deserve losing a limb.

chop off a complete strangers arm, but they come back to kill you in 30 years

The stranger comes back to try to kill you — whether they succeed against a prepared, fully-functional opponent is a separate question entirely.
You retain both arms and full physical capability for thirty years, with ample time to prepare for what is coming.
Strategic target selection — an elderly person, for instance — means the returning attacker arrives in their eighties with one arm.
Thirty years is a full life by most measures, and you spend none of it managing a permanent disability.