Is the hacked client itself safe? Yes. Your PC may say it is a trojan horse, but it's not. (Your pc wants to take precautions so that you don't get hacked. If you download anything from the web, it will probably do this because it is an "Unknown Source"). If you don't feel safe getting rid of your virus protection to keep your PC safe, that's ok, and no one will judge you.
If you use it on servers, you might get banned, as it is a client that gives you an unfair advantage like reach, Killaura, anti knockback, etc. Some features may be ok to use, but since it's a hacked client most hacks on there give you an unfair advantage, so the hacks you can use on there without getting banned are limited. If you do use unfair advantages, I advise you only use this client in your single-player worlds.