Actually, "don't eat it" is the answer. The reason is ground meat is in fact garbage, a mix of stuff that's otherwise unsellable, left in possibly unsanitary conditions until there was enough of it to make ground meat from. People get food poisoning from ground meat, not from steak or cutlets (usually) and that's for a reason: bacteria accumulate on the surface of meats, not on the insides but in ground meat they disperse throughout the product.
So it's not "filler" I'd worry about (factory produced ground meat is probably fatty western meat mixed with fat free - and probably otherwise inedible - meat from, say, Botswana - not in itself a toxic mix) but the way it was made.
Unfortunately, grinding meat is not something you can do yourself unless you own specialized equipment (a blender or a food processor turns meat into a pulp, which is quite different from grinding it). Until you can, it's probably best to avoid it. Even grass fed organic meat makes for pretty unsavory ground meat if it's produced in unsanitary conditions (and it usually is).