There are quite a few tribunals set up, former international criminal tribunal for Yugoslavia and other ad hoc tribunals. Famous jurists, I believe, such as H.LA have argued that international law is not law because it has no legislature (might be wrong on that ages since jurisprudence) but it can be argued that there are jus cogens norms pervading international law. In the ICJ decision of Continental Shelves and under the Statute of the ICJ, customary international law is authoritative.
Anyway, some argue that Jus Cogens are the highest form of law, and all countries recognise since Nuremberg etc that war crimes are repugnant to mankind, thus jus cogens and override other laws. Thus, some argue domestic courts, such as HL, have a the jurisdiction to try them to protect the greater good. It is inherent in international law from various decisions. Therefore, they can try war crimes and champion international HR. See Pinochet case
This is an extentionally complex area of law because often those who commit crimes against humanity are heads of state, thus have sovereign immunity so tribunals arguably cannot have jurisdiction
The ICJ is aimed at land disputes, look at Congo v. Belgium to see how much controversy their jurisdiction over crimes against humanity caused.