There is no thin line. It's a flippin HUGE line.
Without modders you would not have games like Counter Strike. Without hackers you wouldn't have cheating. HUGE DIFFERENCE.
Hackers do not edit game files so much, they create files external to the game which usually get injected into the executable file when the game is launched, there are various methods however. The large majority of Hackers do not even create anything, they go to some lame site, pay $5 and download these hacks.
Modders change things, they change things and make them publicly available (usually) or just for themselfs, such as new skins and weapon models. This is called
Modifying files to enhance\change gameplay.
Hackers create external applications to alter memory adress's within the game, to change statistics and data that effects the ingame Player so they have an unfair advantage over everyone else. This can be as simple in some peoples eyes as have a larger FOV, making smoke totally seethrough, etc.
There is a large line between the two.
I tell you what, go tell a Modder he is a hacker. His reaction will indicate the huge difference between the two.
What you are saying is like saying a Terrorist is the same a US/British Soldier because they both Kill people. Just no.