Mafia: general discussions.

However, in previous Mafia games, players recognized this, and it didn't always help them win.

It's an overpowered ability for the town as a whole, not specifically for the Thief. The town was already strong.

But even so, if she stole it, I don't think I should have been able to use it.

If Hatnik had been blocked, for example, or if he hadn't wanted to use Night Defense, then you could have used it. But if he uses it, then you can't use it. Although, yes, I overcomplicated things here; you still shouldn't have been able to use Night Defense.