Being kind to kaafirs with the intention of da'wah

Praise be to Allah.  

Shaykh Muhammad ibn 'Uthaymeen (may Allah have mercy on him) said: "Undoubtedly the Muslim is obliged to hate the enemies of Allah and disavow himself of them, because this is the way of the Messengers and their followers. Allah says:

{قَدْ كَانَتْ لَكُمْ أُسْوَةٌ حَسَنَةٌۭ فِىٓ إِبْرَ‌ ٰهِيمَ وَٱلَّذِينَ مَعَهُۥٓ إِذْ قَالُوا۟ لِقَوْمِهِمْ إِنَّا بُرَءَ‌ ٰٓؤُا۟ مِنكُمْ وَمِمَّا تَعْبُدُونَ مِن دُونِ ٱللَّهِ كَفَرْنَا بِكُمْ وَبَدَا بَيْنَنَا وَبَيْنَكُمُ ٱلْعَدَ‌ ٰوَةُ وَٱلْبَغْضَآءُ أَبَدًا حَتَّىٰ تُؤْمِنُوا۟ بِٱللَّهِ وَحْدَهُۥٓ}  

'Translation' {Indeed there has been an excellent example for you in Ibraaheem (Abraham) and those with him, when they said to their people: Verily, we are free from you and whatever you worship besides Allah, we have rejected you, and there has started between us and you, hostility and hatred for ever until you believe in Allah Alone} [al-Mumtahanah from the verse:4]. 

{لَا تَجِدُ قَوْمًۭا يُؤْمِنُونَ بِٱللَّهِ وَٱلْيَوْمِ ٱلْءَاخِرِ يُوَآدُّونَ مَنْ حَآدَّ ٱللَّهَ وَرَسُولَهُۥ وَلَوْ كَانُوٓا۟ ءَابَآءَهُمْ أَوْ أَبْنَآءَهُمْ أَوْ إِخْوَ‌ ٰنَهُمْ أَوْ عَشِيرَتَهُمْ ۚ أُو۟لَـٰٓئِكَ كَتَبَ فِى قُلُوبِهِمُ ٱلْإِيمَـٰنَ وَأَيَّدَهُم بِرُوحٍۢ مِّنْهُ}

'Translation' {You (O Muhammad) will not find any people who believe in Allah and the Last Day, making friendship with those who oppose Allah and His Messenger (Muhammad), even though they were their fathers or their sons or their brothers or their kindred (people). For such He has written Faith in their hearts, and strengthened them with Rooh (proofs, light and true guidance) from Himself} [al-Mujaadilah from the verse:22]. 

Based on this, it is nor permissible for a Muslim to feel any love in his heart towards the enemies of Allah, who are also in fact his enemies. Allah says:

  {يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ لَا تَتَّخِذُوا۟ عَدُوِّى وَعَدُوَّكُمْ أَوْلِيَآءَ تُلْقُونَ إِلَيْهِم بِٱلْمَوَدَّةِ وَقَدْ كَفَرُوا۟ بِمَا جَآءَكُم مِّنَ ٱلْحَقِّ}

'Translation' {O you who believe! Take not My enemies and your enemies (i.e. disbelievers and polytheists) as friends, showing affection towards them, while they have disbelieved in what has come to you of the truth} [al-Mumtahanah from the verse:1]. 

But if a Muslim treats them kindly hoping that they may become Muslims, there is nothing wrong with that, because this comes under the heading of softening their hearts towards Islam.

But if he loses hope of them becoming Muslim, he should treat them in the manner that they deserve. This is explained in detail in the books of the scholars, especially Ahkaam Ahl al-Dhimmah by Ibn al-Qayyim (may Allah have mercy on him) (From Majmoo' Fataawa Ibn 'Uthaymeen, [3/31]).