Why oppressing people is a despicable crime in Islam?

SHAFAQNA – Imam Ali (AS) said: Swear to God, if I sleep on Sa’adaan barbs (triple barbs) all night, or I am pulled here and there whilst chained and handcuffed, I prefer to meet Allah (SWT) and the Prophet (PBUH) on the Day of Judgment (in this condition), rather than meeting them whilst I had oppressed some servants of God, and/or had stolen from the public wealth! How can I oppress someone for my own soul which is moving towards getting old and being decayed, and is going to remain in the soil (grave) for a long time [1]?

[1] Nahjul Balaghah, Sermon 224.