Some Believers stayed away from Battle because They were Lazy
Allah tells;
وَاخَرُونَ اعْتَرَفُواْ بِذُنُوبِهِمْ خَلَطُواْ عَمَلً صَالِحًا وَاخَرَ سَيِّيًا عَسَى اللّهُ أَن يَتُوبَ عَلَيْهِمْ إِنَّ اللّهَ غَفُورٌ رَّحِيمٌ
And (there are) others who have acknowledged their sins, they have mixed a deed that was righteous with another that was evil. Perhaps Allah will turn unto them in forgiveness. Surely, Allah is Oft-Forgiving, Most Merciful.
After Allah explained the characteristics of the hypocrites who stayed away from battle because they sought to avoid it out of denial and doubt, He then mentioned the disobedient who stayed away from Jihad due to laziness and preferring comfort, even though they truly believed,
وَاخَرُونَ اعْتَرَفُواْ بِذُنُوبِهِمْ
And others who have acknowledged their sins,
These people admitted their error to themselves and their Lord. They had performed good deeds before, as well as, this evil deed that they committed. For them there was forgiveness and pardon of Allah.
This Ayah is general, covering all sinners who combine good and evil deeds, thus becoming partly impure, even though it was revealed about some people in specific.
Ibn Abbas said that,
وَاخَرُونَ
(And (there are) others),
refers to Abu Lubabah and some of his friends who stayed away from the battle of Tabuk and the Messenger of Allah. When the Messenger of Allah returned from that battle, this group, Abu Lubabah and five, seven or nine with him, tied themselves to the pillars of the Masjid and refused to let anyone untie them except the Messenger of Allah. When this Ayah was revealed,
وَاخَرُونَ اعْتَرَفُواْ بِذُنُوبِهِمْ
(And (there are) others who have acknowledged their sins), the Messenger of Allah untied them and pardoned them."
Al-Bukhari recorded that Samurah bin Jundub said that the Messenger of Allah said to us,
أَتَانِي اللَّيْلَةَ اتِيَانِ فَابْتَعَثَانِي فَانْتَهَيَا بِي إِلَى مَدِينَةٍ مَبْنِيَّةٍ بِلَبِنِ ذَهَبٍ وَلَبِنِ فِضَّةٍ فَتَلَقَّانَا رِجَالٌ شَطْرٌ مِنْ خَلْقِهِمْ كَأَحْسَنِ مَا أَنْتَ رَاءٍ وَشَطْرٌ كَأَقْبَحِ مَا أَنْتَ رَاءٍ
قَالَاا لَهُمْ اذْهَبُوا فَقَعُوا فِي ذَلِكَ النَّهْرِ فَوَقَعُوا فِيهِ ثُمَّ رَجَعُوا إِلَيْنَا قَدْ ذَهَبَ ذَلِكَ السُّوءُ عَنْهُمْ فَصَارُوا فِي أَحْسَنِ صُورَةٍ
قَالَاا لِي هَذِهِ جَنَّةُ عَدْنٍ وَهَذَا مَنْزِلُكَ
قَالَاا وَأَمَّا الْقَوْمُ الَّذِينَ كَانُوا شَطْرٌ مِنْهُمْ حَسَنٌ وَشَطْرٌ مِنْهُمْ قَبِيحٌ فَإِنَّهُمْ خَلَطُوا عَمَلًا صَالِحًا وَاخَرَ سَيِّيًا تَجَاوَزَ اللهُ عَنْهُم
Last Night, two (angels) came to me (in a vision) and took me to a city, built with bricks made of gold and silver. We met some men who, part of their bodies were as handsome as you ever saw and the part as ugly as you ever saw.
The two (angels) ordered these men to go to a river and submerge themselves in it; they did that and came back to us, and the ugliness went away from them, thus becoming the most beautiful form.
The two said to me, `This is the garden of Eden, and this is your residence in it.'
The two said, `As for the men who had part of their body handsome and part ugly, they have mixed a deed that was righteous with another that was evil. Allah has pardoned them.'
Al-Bukhari recorded this Hadith in a short form upon the explanation of this Ayah