Saddam was at the time essentially a CIA plant dictator put into place to stop the spread of the Islamic revolution and the rise of Arab Socialism. For Iran, Saddam was gassed up by the US to act as a distraction for Iran and to draw both countries into a long, drawn out conflict with the aim to drain Iran, and push Iraq further into a role of an American Neo-colony.

In all, Saddam was a terrible person and leader, but that neither justifies nor excuses the American invasion and subsequent slaughter of Iraqis. Especially seeing as how America placed Saddam into his position of power. They created their own boogeyman. Saddam deserves critical support against the US, but he was neither a socialist leader, nor someone that leftists should support.

Long story short, the US encouraged him to do it in hopes of reversing or at least stopping the Islamic Revolution from spreading. The Americans and the Europeans armed both sides, including with chemical weapons to make sure that it would be as big a bloodbath as possible.