Hitler was excommunicated automatically, during his lifetime, many, many times.
Among other things, his troops, almost certainly on his orders, shot hundreds of priests in Poland after the war started.
That's good for hundreds of excommunications right there.
In Hitler's case, it's kind of irrelevant whether he was excommunicated once or a million times -- he was not a communicant in the first place. (As far as is known, he did not attend Mass or receive the Eucharist after his middle teen years.)
Hitler claiming he "was a Catholic and would remain one" may have been technically true -- an excommunicate Catholic is still a Catholic.
However, he also stated before friends that he was a "pure heathen". That sounds like a formal defection from the Faith, which would make him not a Catholic anymore.
In general, Hitler told people what he wanted them to hear. He lied early and often. Moreover, who is and isn't a Catholic, and what kind of standing they're in, are determined by the Church and her laws, not by the individual's claims. (cf the ladies in St. Louis claiming to be "Catholic priests").
As far as him going to purgatory, you seem to think that Catholics believe that anyone who dies a Catholic goes to purgatory. Wrong. Someone who dies in the state of grace goes to purgatory. Someone guilty of millions of murders, his own suicide, and who knows what else, who dies unrepentant of those crimes, is not in the state of grace.