He caused some kittens to die – will his repentance be accepted?
Question

                I live in an apartment building and in my apartment I left one of the windows open. A cat gave birth to four kittens, and I spoke to my father about these kittens and he told me to keep them for a while until the kittens got a little bigger and could fend for themselves, but I only kept them for a short while, until their eyes were open and they had begun to walk, then I threw them out of the apartment and put them on the roof, although I knew that their mother would never be able to find where they had gone. This is what actually happened. 

This happened although I knew the hadeeth of the Messenger (peace and blessings of Allaah be upon him) about the woman who entered Hell because of a cat, but I still left them and went away from that place for three days. 

What has happened now is that after I came back, I found that the kittens are not there, and their mother is looking for them. It seems that they fell into the drains of the building. 

What is the ruling on me in this case, as I could have prevented that and saved these kittens, and that would not have cost me anything, and I also went against my father’s advice? Am I a sinner? What should I do to expiate for my sin? Please advise me, may Allaah have mercy on you and reward you. I feel very depressed and I cannot enjoy life. I am just waiting for death so that I can find some relief from these feelings. All the time I think about these kittens and I think they never harmed me but I killed them, so I will go to Hell like that women in the hadeeth. 

I have become depressed and have no enthusiasm for anything I do. I do not find any joy in anything. I wish that I could turn back the clock. I feel so lost and helpless. I will never forget this incident and the pain it has caused me, in this world or in the Hereafter.
Praise be to Allah.Allaah has 
    enjoined kindness in all things, as it says in the hadeeth: “Allaah is Kind 
    and He loves kindness in all things.” Narrated by al-Bukhaari (5678) and 
    Muslim (2165). 
Muslim 
    (2592) narrated that Jareer (may Allaah be pleased with him) said: I heard 
    the Prophet  (peace and blessings of Allaah be upon him) say: “Whoever 
    is deprived of kindness is deprived of goodness.” 
Remember 
    that whoever has any kind of animals must protect them and give them food 
    and water. It is not permissible for him to expose them to danger. In the 
    hadeeth that you referred to, the Messenger  (peace and blessings of 
    Allaah be upon him) told us that a woman entered Hell because of a cat that 
    she detained and did not feed it or let it eat from the vermin of the earth, 
    i.e., insects. Narrated by al-Bukhaari (2365) and Muslim (904). 
It was 
    narrated that ‘Abd-Allaah ibn Mas’ood (may Allaah be pleased with him) said: 
    We were with the Messenger of Allaah  (peace and blessings of Allaah be 
    upon him) on a journey, and he went to relieve himself. We saw a bird with 
    two chicks and we took away her chicks, and the bird started flapping her 
    wings. The Prophet  (peace and blessings of Allaah be upon him) came and 
    said: “Who has upset this bird by taking her children? Give her children 
    back to her.” Narrated by Abu Dawood (2675); classed as saheeh by Shaykh 
    al-Albaani in Saheeh Abi Dawood. 
The point is 
    that whomever Allaah has given any of these animals to look after, such as 
    if they are in his care or he is raising them and keeping them for some 
    need, then he must look after them and treat them kindly. 
Based on 
    that, then you have to repent to Allaah for several reasons: 
1 – 
    Disobeying your parents whom Allaah has commanded you to honour and obey so 
    long as there is no sin involved in doing so. 
2 – These 
    kittens were in your care but you failed to look after them, and that led to 
    their death. 
3 – You 
    upset their mother by separating her and her children, then her little ones 
    died. 
Undoubtedly 
    the feelings of sin that you describe indicate that your heart is alive and 
    that you have sincere feelings of regret for what you have done. 
But perhaps 
    that has gone too far, such that you feel despair, and this undoubtedly is 
    not permissible, as Allaah says (interpretation of the meaning): 
“Say: O 
    ‘Ibaadi (My slaves) who have transgressed against themselves (by committing 
    evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah 
    forgives all sins. Truly, He is Oft‑Forgiving, Most Merciful”
[al-Zumar 
    39:53]
What about 
    the vastness of Allaah’s mercy, which encompasses all things? What about 
    Allaah’s acceptance of His slaves’ repentance, even from major sins? 
The Prophet 
     (peace and blessings of Allaah be upon him) told us of a man who killed 
    ninety-nine people, then he killed a devoted worshipper, thus completing one 
    hundred, then he repented sincerely and Allaah accepted his repentance – as 
    is narrated in al-Saheehayn. What is your sin in comparison to his? 
    He killed one hundred people then repented, and Allaah accepted his 
    repentance. So you have no right to despair of the forgiveness and mercy of 
    Allaah. Do righteous deeds and always remember the words of Allaah , may He 
    be exalted: “Verily, the good deeds remove the evil deeds” [Hood 11:114]. 
And Allaah knows best.