The "should not" means that it is conditional.
This is not a correct understanding of this passage; it's not your fault, but rather the translation you chose. That's the problem with translations: they can't (or
don't, as is the case here) always communicate clearly what the Greek says (and, in this case, what it does
not say).
The phrase you quoted ("should not perish") is translating a
subjunctive clause, not a conditional clause. Conditional clauses in Greek are marked by conditional particles (εἰ, ἄν, and ἐάν), all of which are absent from this passage. Subjunctives function in many ways depending on the grammatical context. This subjunctive here in John 3:16 is marking not a conditional clause, but a
purpose clause, which is indicated by the conjunction ἵνα. Because of this, the subjunctive here would be better rendered "
will not perish" because it is a purpose clause, not a conditional clause. The ESV, KJV, and NKJV have, in my opinion, failed to render this in its fullest clarity. "Should" in English (which, depending on context, has its own variety of functions) is too vague a term to use with the subjunctive case here.
