I’m sorry for the snark, but Major General Smedley Butler offers a generic explanation for wars in general in his 1935 essay War is a Racket.

But for the Iraq war specifically (a part from the racketeering) why many people think it was the wrong decision is in large part based on the lies and deceptions that were used to justify the invasion.

At the time many people believed those lies and thought they were justifiable reasons for the invasion. When it later turned out there were no weapons of mass destruction, that ties of the Ba'ath party to terrorist organizations were none, that the USA imposed government was corrupt and offered little benefits to regular people over Saddam’s dictatorship, etc. etc. When this all became common knowledge, on top of all the war crimes, the torture scandals, the massacres, after Chelsea Manning and Julian Assange went to prison for revealing some of those war crimes, many of these people who previously believed the war was justified, changed their opinion of it.