or he is to be killed only there is no chance in Amalek for tchuva ? Please advise who is amelek how to understand it

Amalek was a terribly anti-Semitic nation, but our rabbis teach that even Amalek can do teshuva, and their descendants even studied Torah in Bnei Brak (Sanhedrin 96b)! Theoretically, in ancient history (Amalek has not been around for 2,500 years already!) one would have had to kill them (see below for the reasons why), but if they already converted, they are "in". For more on who they are, why the Torah has no toleration for them, see my previous answers: http://old.yeshiva.org.il/ask/nihul/Eadmin_search_ask.asp?E=&srch=1&q=amalek