The heavy nūn does not turn a passive verb into an active one. It emphasises the passive proposition: someone or something will indeed be feared.
PronounPassive with heavy nūn
هُوَلَيُخَافَنَّ
هُمَا (م)لَيُخَافَانِّ
هُمْلَيُخَافُنَّ
هِيَلَتُخَافَنَّ
هُمَا (ف)لَتُخَافَانِّ
هُنَّلَيُخَفْنَانِّ
أَنْتَلَتُخَافَنَّ
أَنْتُمَا (م)لَتُخَافَانِّ
أَنْتُمْلَتُخَافُنَّ
أَنْتِلَتُخَافِنَّ
أَنْتُمَا (ف)لَتُخَافَانِّ
أَنْتُنَّلَتُخَفْنَانِّ
أَنَالَأُخَافَنَّ
نَحْنُلَنُخَافَنَّ
لَيُخَافَنَّ — someone/something will certainly be feared.
By contrast, لَيَخَافَنَّ means “he will certainly fear.” The prefix vowel distinguishes voice, while the doubled nūn conveys emphasis in both.