Imam al-Ghazāli, articulates in his magnus opus: the Ihyā’ Ulūm al-Dīn (The Revival of the Religious Sciences) , that sins are of two types; sins in which Man transgresses the rights he owes to God, and sins in which Man transgresses the rights which God has granted to creation (Huqūq’ul Ibād).
Furthermore, sins committed against others are much more difficult to have forgiven than those committed against God.
But why is this so? Why are the rights of others so serious a matter?
It is because that which is between man and God [alone], unless it is idolatry (ṣhirk), will always have greater hope for pardon given our Creator is The Most Merciful.
In fact, our account on the day of reckoning is in danger of not only having all virtues exhausted but also having sins heaped upon because of neglecting the rights of others.
It has been transmitted in a tradition narrated by Abu Ḥurairah that The Messenger of Allāh (PBUH) asked: “Do you know who is bankrupt?” They (the Companions of the Prophet) replied: “A bankrupt man amongst us is one who has neither dirham with him nor wealth.” He (PBUH) said: “The poor of my Ummah would be he who would come on the Day of Resurrection with prayers and fasts and Zakāt but since he hurled abuses upon others, brought calumny against others, unlawfully consumed the wealth of others, shed the blood of others and beat others, his virtues would be credited to the accounts of others (who suffered at his hands). And if his good deeds fall short to clear the account, then the other’s sins will be entered in (his account) and he will be thrown in the Fire of Hell.”
In fact the concept of Huqūq’ul Ibād extends beyond inter-human relations to encompass all of creation including animals and also inanimate objects like trees crops and the environment.
The Messenger of Allāh’s (PBUH) words and behaviour make it clear that refraining from physical cruelty towards animals is not enough; abstaining from mental cruelty is equally as important. Even a bird’s emotional distress should be treated seriously. One of The Messenger companions narrates, “We were on a journey and during the Prophet’s absence, we saw a bird with its two chicks; we took them. The mother bird was circling above us in the air, beating its wings in grief. When The Messenger of Allāh returned he said, “Who has hurt the feelings of this bird by taking its chicks? Return them to her.” 
Another tradition states that faith has more than seventy branches. The highest is to bear witness that there is no God but Allāh and Muḥammad (PBUH) is the Messenger of Allāh and the least one is to remove some obstacle (stone, wood, thorn, etc.) from the path.
We often talk of spiritual bankruptcy in society. Perhaps this is the consequence of us neglecting in our duty to fulfill the rights of others.
So let us strive to please God by serving His creation, as vast as it is, for in serving creation do you find the Creator.
And Allāh knows best.
Hāroon Ibn Ebrāhīm Sīdat.
 Ihyā’ Ulūm al-Dīn: kitab al-Tawbah
 Musnad Ahmad VI 240
 Sharhu of Imam Nawawi