Zina is a great sin which is so great that it has been mentioned in the Qu'ran along with Shirk and Murder.
Allah says in the Qu'ran:
وَالَّذِينَ لَا يَدْعُونَ مَعَ اللَّهِ إِلَٰهًا آخَرَ وَلَا يَقْتُلُونَ النَّفْسَ الَّتِي حَرَّمَ اللَّهُ إِلَّا بِالْحَقِّ وَلَا يَزْنُونَ ۚ وَمَنْ يَفْعَلْ ذَٰلِكَ يَلْقَ أَثَامًا
“And those who invoke not any other ilaah (god) along with Allaah, nor kill such person as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse”
Al-Safaareeni (may Allah have mercy on him) said:
Zina is the most serious of major sins after shirk and murder.
Ghidha’ al-Albaab, 2/305
‘Abd-Allaah ibn Mas’ood said:
“I asked the Messenger of Allaah (peace and blessings of Allaah be upon him), ‘Which sin is worst in the sight of Allaah?’ He said, ‘To make any rival to Allaah, when He has created you.’ I asked, ‘Then what?’ He said, ‘To kill your child for fear that he will eat with you.’ I asked, ‘Then what?’ He said, ‘To commit zinaa with the wife of your neighbour.’”
(Reported by al-Bukhaari, 8/492; Muslim, 1/90).
As you can see in these verse and hadith, Zina is a sin which comes after Murder.
Allah says in the Qu'ran:
وَمَنْ يَقْتُلْ مُؤْمِنًا مُتَعَمِّدًا فَجَزَاؤُهُ جَهَنَّمُ خَالِدًا فِيهَا وَغَضِبَ اللَّهُ عَلَيْهِ وَلَعَنَهُ وَأَعَدَّ لَهُ عَذَابًا عَظِيمًا
“And whoever kills a believer intentionally, his recompense is Hell to abide therein; and the Wrath and the Curse of Allaah are upon him, and a great punishment is prepared for him”
For Zina,
وَلَا تَقْرَبُوا الزِّنَا ۖ إِنَّهُ كَانَ فَاحِشَةً وَسَاءَ سَبِيلًا
“And come not near to unlawful sex. Verily, it is a Faahishah (i.e. anything that transgresses its limits: a great sin), and an evil way that leads one to hell unless Allaah forgives him”
It clearly states that one is doomed to Hell forever if one kills, but Zina is not that harsh than murder.
The Prophet Muhammad said:
" لَنْ يَزَالَ الْمُؤْمِنُ فِي فُسْحَةٍ مِنْ دِينِهِ، مَا لَمْ يُصِبْ دَمًا حَرَامًا"
“A man will continue to be sound in his religion so long as he does not shed blood which it is forbidden to shed.”
Narrated in Bukhari 6862
This means he is a disbelieving if he kills innocently. However, with Zina, the Prophet said that one is a disbeliever whilst commiting Zina:
Al-Bukhaari narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said:
“No adulterer is a believer at the time when he is committing adultery; no thief is a believer at the time when he is stealing; no drinker of wine is a believer at the time when he is drinking it.”
Narrated by al-Bukhaari (5578) and Muslim (57)
To summarise, Murder is a bigger Sin than Zina but Zina is still one of the worst sins.
And Allah knows best.
