faithfreedom.com is a zionist propaganda website... Kafir means non-believer... its the same as nastik in hindu religion... so you should not be offended by the word.... its not a slang.....
about that site.. its a propaganda to dupe people... I will give you one example... If you read just one sentence of geeta Krishna tells Arjun to kill his relatives but there is a context for which its valid...
Likewise whatever is written in Koran is on specific context.... Unless you know the context you cannot say that Koran simply tells to kill non-believers....
So why not get an English translated version of Koran & find out why such thing is written & for what context....