Treason. The answer is high treason.

The very short version is

After the Iranian Hostage Crisis, the US placed an arms embargo on Iran so they legally couldn’t sell arms to Iran

Reagan wanted to fund the Contra rebels in Nicaragua, however Congress had prevented him from directly funding them so they had to find some way of doing that off the books.

So since Iran wanted to buy weapons, they sold them some weapons so they could turn around and use that money to fund the Contras and circumvent Congress.

They also tried to justify it because they were hoping Iran would influence Hezbollah to release hostages they had taken in Lebanon (which kind of worked, the hostages were released, but Hezbollah pretty much immediately took more hostages, so not really)

This glosses over a lot of details of course.

Why would the US arm people who hate us?

Wait, which time?

You don’t know of the great patriot Ollie north?