ASP.NET MVC 5 how to delete a user and its related data in Identity 2.0

Update for ASP.NET Core 2.0 - hope this saves someone a bit of time

ApplicationDbContext context, 
UserManager<ApplicationUser> userManager, 
ApplicationUser user

var logins = await userManager.GetLoginsAsync(user);
var rolesForUser = await userManager.GetRolesAsync(user);

using (var transaction = context.Database.BeginTransaction())
{
    IdentityResult result = IdentityResult.Success;
    foreach (var login in logins)
    {
        result = await userManager.RemoveLoginAsync(user, login.LoginProvider, login.ProviderKey);
        if (result != IdentityResult.Success)
            break;
    }
    if (result == IdentityResult.Success)
    {
        foreach (var item in rolesForUser)
        {
            result = await userManager.RemoveFromRoleAsync(user, item);
            if (result != IdentityResult.Success)
                break;
        }
    }
    if (result == IdentityResult.Success)
    {
        result = await userManager.DeleteAsync(user);
        if (result == IdentityResult.Success)
            transaction.Commit(); //only commit if user and all his logins/roles have been deleted  
    }
}

I think the classes you're looking for are the UserManager and the RoleManager. In my opinion they are the better way instead of going against the context directly.

The UserManager defines a method RemoveFromRoleAsync which gives you the ability to remove the user (identified by his key) from a given role. It also defines several Find methods, such as FindAsync, FindByIdAsync, FindByNameAsync, or FindByEmailAsync. They all can be used to retrieve a user. To delete a user you should use the DeleteAsync method which accepts a user object as a parameter. To get the roles a user is member of Identity gives you the GetRolesAsync method where you pass in the ID of the user. Also I see that you're trying to remove a login from a user. For this purpose you should use the RemoveLoginAsync method.

All in all your code would look similar to the following one:

// POST: /Users/Delete/5
[HttpPost, ActionName("Delete")]
[ValidateAntiForgeryToken]
public async Task<ActionResult> DeleteConfirmed(string id)
{
  if (ModelState.IsValid)
  {
    if (id == null)
    {
      return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
    }

    var user = await _userManager.FindByIdAsync(id);
    var logins = user.Logins;
    var rolesForUser = await _userManager.GetRolesAsync(id);

    using (var transaction = context.Database.BeginTransaction())
    {
      foreach (var login in logins.ToList())
      {
        await _userManager.RemoveLoginAsync(login.UserId, new UserLoginInfo(login.LoginProvider, login.ProviderKey));
      }

      if (rolesForUser.Count() > 0)
      {
        foreach (var item in rolesForUser.ToList())
        {
          // item should be the name of the role
          var result = await _userManager.RemoveFromRoleAsync(user.Id, item);
        }
      }

      await _userManager.DeleteAsync(user);
      transaction.Commit();
    }

    return RedirectToAction("Index");
  }
  else
  {
    return View();
  }
}

You'll need to adjust this snippet to your needs, because I don't have an idea how your IdentityUser implementation looks like. Remember to declare the UserManager as needed. An example how you could do this can be found when you create a new project in Visual Studio using Individual Accounts.