Compare:
"If one is to boast of flesh and blood the Jews are actually nearer to Christ than we are...If we really want to help them, we must be guided in our dealings with them not by papal law but by the law of Christian love. We must receive them cordially, and permit them to trade and work with us, that they may have occasion and opportunity to associate with us, hear our Christian teaching, and witness our Christian life. If some of them should prove stiff-necked, what of it? After all, we ourselves are not all good Christians either."
"There is no other explanation for this than the one cited earlier from Moses - namely, that God has struck [the Jews] with 'madness and blindness and confusion of mind' [Deuteronomy 28:28]. So we are even at fault in not avenging all this innocent blood of our Lord and of the Christians which they shed for three hundred years after the destruction of Jerusalem, and the blood of the children they have shed since then (which still shines forth from their eyes and their skin). We are at fault in not slaying them."
Luther had already split from the Catholic Church by 1523. So what exactly happened from 1523-1543 that led to such a drastic reversal in Martin Luther's thinking?
More can be written on this, but you might be interested by 'Why did Martin Luther write his treatise "The Jews and their Lies"? Why did he hate Jews so much?' by /u/sunagainstgold . They provided other answers below that top-level one on related issues.
But this is not to discourage discussion. More questions, data, and debate are welcome.
and the blood of the children they have shed since then (which still shines forth from their eyes and their skin).
What is this supposed to mean? Taken literally it sounds like something from a horror movie...