If Saddam was a murderous tyrant who killed tens of thousands, doesn't that make the USA an accessory?

I often hear from US citizens that Iraq was inaded to remove Saddam, as he was a murderous dictator who macred his own citizens. Since most of that activity went on in the 80's, when he was a US ally against Iran, and the US was selling him billions of dollars in arms (not to mention sarin), doesn't that make the USA an accessory to those macres? Also, where was the USA's concern for the good citizens of Iraq then? Or did they only start being concerned when it became politically expedient to do so?