What actual Catholic sin did he commit anyway? Keeping in mind that murder in Catholic theology is an unlawful killing (at least thats what they tell me when I bring up all those mercilessly slaughtered by the Hebrews in their bible) and everything Hitler did was fully legal in Germany, him being der fuhrer and all.

Good grief this is awful. The unlawfulness that they are talking about is whether it an action corresponds to the divine law. No Catholic theologian holds that merely because an action is endorsed by the state, it therefore becomes moral. See, for example, St Thomas Aquinas:

"...laws may be unjust through being opposed to the Divine good: such are the laws of tyrants inducing to idolatry, or to anything else contrary to the Divine law: and laws of this kind must nowise be observed, because, as stated in Acts 5:29, "we ought to obey God rather than man."