We preserve food so that we can eat it later without it going bad. We process things in cans or glass jars, we dry (dehydrate) things like beef to make beef jerky, we pickle things, we freeze things...
Beef is not bad for your system in small amounts. You will also not want to eat beef that is not properly cooked.
It sounds like it is going bad. It is probably best to replace it.
Maybe not. It depends what made the beef "bad".
they don't
Eventually, yes. However, it's usually dried, which slows down the decay processes tremendously (beef jerky can be left out for months without going bad; try that with a steak and it will be stinking by the end of the third day or so at most).
Not necessarily, browning of beef while in the freezer or fridge just means that it is not as fresh as it could be. If it has a green tinge to it you know it is now bad.
If beef taste like fish then throw it out.
Yes, it is. You should not eat bad beef because it causes potentially fatal food-borne illness.
You should use beef stock two weeks after opening.
look at the label
Not necessarily. MAC itself is not bad for your skin, but ANY makeup brand can & will age you if you're not taking care of yourself i.e. not washing all your off before going to bed, not moisturizing your skin, or staying out in the sun without SPF