What is the name of 1970s or early 1980s sci-fi movie in which a kid finds weapon from outer space that he fits onto his arm and ends up becoming evil?

The movie you're thinking of is likely "The Day of the Triffids" (1962), though it's not exactly a kid finding a weapon.

Here's why it might fit your description:

* The protagonist isn't a child: The main character, Bill Masen, is an adult who is temporarily blinded. He finds a weapon, but it's not a weapon in the traditional sense. It's a device that allows him to see in the dark.

* Evil Consequences: The device allows him to see in the dark, but it also seems to have a mind of its own and eventually leads him to make some morally questionable choices.

While it doesn't fit your description perfectly, it's the closest match I can find based on your clues. The weapon being on the arm and the character turning evil are both present, even if the character isn't a child.

If you can remember any more details, like the name of the weapon, the location where it's found, or any other specific scenes, it might help to narrow down the movie you're looking for.