https://www.jewishvirtuallibrary.org/the-crusades
https://www.chabad.org/library/article_cdo/aid/2617029/jewish/The-Bloody-Crusades.htm
“Although compared to later tragedies the loss of Jewish life was relatively small, with the main devastation occurring in but four Rhineland towns, the First Crusade has generally been regarded by Jews as a disaster of epic proportions. The period of counting the Omer, between Pesach and Shavuos, when the massacres occurred, became fixed in Jewish law as a time for mourning. A prayer commemorating the martyrs, Av HaRachamim, was added to the Sabbath morning services and is recited weekly, except on joyous occasions. Several Kinnos were composed remembering these events and became part of the Tisha B’Av service. There are several reasons why the First Crusade has been given such prominence, while other, seemingly far greater tragedies have not:
The four towns destroyed were major Torah centers of Ashkenazic Jewry. Although Jews resettled and rebuilt these communities, and Ashkenazic Torah centers flourished, the greatness of these cities’ martyred scholars was lost forever – a theme that appears prominently in the Kinnos.
The Crusades set a dangerous precedent — the rise of organized, popular, anti-Jewish uprisings. Although both the Pope and the local authorities were generally opposed to the Crusaders’ excesses in Germany, these leaders’ hostility to Jews caused them to remain apathetic to Jewish suffering, thus they generally did not intervene. After the First Crusade, instances of mob persecution occurred regularly. Therefore, the Crusades can be seen as the source for much of subsequent Christian persecution. In keeping with the traditional Jewish viewpoint, that the beginning of a tragedy is noted, the events of the Crusades are commemorated.”
The Crusader massacres of Jews (and Christians and Muslim civilians) was unjustified.
This also was strategically a sapping of resources.
However you bringing it up indicates that you may see it only from the victim mentality and not looking at what happened prior.
In the first Crusade Jews aided the Muslims against the Christians. Haifa was Jewish controlled and fought off the Christians.
This anti-Jewry also had its roots in the fact that Jews aided Muslims in taking over Jerusalem, Syria, North Africa and Spain. This is because the Jews saw the monotheistic Muslims (and Islam had not yet been clearly defined in the first 150 years of Arab empire) as ideologically closer to them. Note also that the Copts and some Syriacs also did support the Muslims as they were Semitic speakers. But that was in Egypt.
In the first Crusade - During this time, a full thousand years after the fall of the Jewish state, there were Jewish communities all over the country. Fifty of them are known and include Jerusalem, Tiberias, Ramleh, Ashkelon, Caesarea.
Jews fought side-by-side with Muslim soldiers to defend Jerusalem against the Crusaders.
AND, you forget Christians stood up and attempted to protect the neighboring Jews. In the German city of Trier, the local bishop attempted to protect the Jews —> reference Jonathan M. Elukin, Living Together, Living Apart: Rethinking Jewish-Christian Relations in the Middle Ages (Princeton, NJ: Princeton UP, 2007),
Other German cities had similar experiences, with some towns such as Mainz having the local burghers fight against the incoming crusaders.
Another German town, Cologne, hid all the local Jews among their Christian neighbors during the Jewish holiday of Shavuot, spending the remainder of the holiday with the Christian acquaintances.
Shrug off the victim mentality when looking at history - warts and all.