Many of the relevant UN websites for this section are those related to International Criminal Law. Links are provided for appropriate websites. The principal instruments and protocols contain the limitation and indeed prohibitions on the use of the death penalty.
www.ohchr.org - The Office of the High Commissioner for Human Rights
Genocide and war crimes
These websites contain more information on the law relating to genocide.
https://unictr.irmct.org - The International Criminal Tribunal for Rwanda (legacy website)
www.icty.org/ -The International Criminal Tribunal for the former Yugoslavia
https://www.icc-cpi.int - The International Criminal Court
www.ohchr.org/EN/ProfessionalInterest/Pages/CrimeOfGenocide.aspx - This link goes to the text of the Convention on the Prevention and Punishment of the Crime of Genocide
unmict.org - UN Mechanism for International Criminal Tribunals