'If capital punishment actually deterred people from committing serious crimes, then it would be justified. But as it does not have that deterrent effect, it is not a justifiable practice.' Which formal fallacy is committed in the above argument?

The argument denies the antecedent.

Its form is: if capital punishment deterred crime (P), it would be justified (Q).

The second premise asserts not-P, that it has no deterrent effect.

From not-P it concludes not-Q, that it is not justifiable.

Denying P cannot establish not-Q, since Q may hold on other grounds.

Concluding not-Q from a conditional plus not-P is denying the antecedent.