Nulled theme basically mean cracked/hacked. The distributors of such themes often hide popups/ads inside to earn money, which you can't see until a user complains about it or you check the website on google speed test for example where you see the image of the website, they aren't secure at all, and it is not worth using them.
Expected behavior of nulled themes and plugins:
Hidden ads/popups/links which benefit the distributor, bad performance in most of the cases, sometimes malware and shells are being injected in the theme, in order to track the information.
Saving 40$ for a theme will waste your time in debugging the nulled theme, living in fear if the developer will take legal actions against you for it, lack of updates, lack of support for it and other features which are theme related and last but not least you support a bad industry and split on many hours of developing a theme.
Simply don't! : )