It is reported from Ibn Abbas (may Allah be pleased with them both) that he (peace and blessings be upon him) said: "The most hateful of people to Allah are: the one who commits irreligion in the Haram, the one who introduces the ways of Jahiliyyah into Islam, and the one who unjustly sheds someone's blood."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15902