If we delete the user from User information list then is it effects the SharePoint list items createdby and modifiedby values related to that deleted user?
I think it will not effect anything because CreatedBy and ModifyBy fields get data from both Active directory and "User Information List" of SharePoint (Which is a hidden list - SharePoint will create an entry in this list when the user added to the site or visits the site for the first time. SharePoint sync this list from user profile when user create/edit/update/delete items).
What happens is when a user is deleted, and that information is pushed to the Site Collection, the user is marked as deleted within the UserInfo table (tp_Deleted = 1). This maintains referential integrity to any references to that user (such as Created By, Modified By, etc.) within that Site Collection.