The sin of one who kills an innocent person having a treaty with the Muslims

Narrated 'Abdullah bin 'Amr:

The Prophet ﷺ said, "Whoever killed a person having a treaty with the Muslims, shall not smell the smell of Paradise though its smell is perceived from a distance of forty years."

حَدَّثَنَا قَيْسُ بْنُ حَفْصٍ، حَدَّثَنَا عَبْدُ الْوَاحِدِ، حَدَّثَنَا الْحَسَنُ بْنُ عَمْرٍو، حَدَّثَنَا مُجَاهِدٌ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو ـ رضى الله عنهما ـ عَنِ النَّبِيِّ ﷺ قَالَ " مَنْ قَتَلَ مُعَاهَدًا لَمْ يَرَحْ رَائِحَةَ الْجَنَّةِ، وَإِنَّ رِيحَهَا تُوجَدُ مِنْ مَسِيرَةِ أَرْبَعِينَ عَامًا ".