Did they had a good cause? Or a bad one? As far as i know the bible preach peace. I understand that at first they were just trying to protect christianity when muslims were attacking the Byzantine Empire but then they rather attacked muslims in the holy land. So what is your opinion about the crusades?
The Crusades were the result of the attacks yes to the Holy Land and the killings of the peregrines that made the trip to visit the Holy places there, but also from the expansion of the muslim armies that at one point conquered most of Spain and parts of Italy.
Yes Italy was partly conquered by muslims, they completely dominated Sicily and parts of Calabria and Puglia, the most Southern parts of the boot.
Recently the Pope canonized 800 saints that were martired at Otranto in 1480 when they were offered the chance of either converting to islam or face execution (beheading) by the muslims that had won the battle.
Does that ring a bell?
With current events?
So we are looking at war events that were triggered as a reaction to outside forces.
It has been shown that in effect the battle of Otranto was the equivalent to the 9/11 of our days.
The known world after the fall of the Roman Empire had lost most of its commerce and devolved into many mini states. This battle sorts of was the catalist for the beginning of the reunification of western Europe, a process that has not ended yet

Were there abuses and injustices commited during this wars?
I would respond, which war has not?
No war can claim that unfortunately. We Catholics pray for peace and by that token war should be our last recourse in settling human affairs.
The important thing to learn about the crusades I believe is summed up in this quote:
“Those who cannot remember the past are condemned to repeat it”
Key on the “remember” which is also equivalent to “learn” “understand”.
Not rely solely on anti religious propaganda that always ascribe the “evils” of the crusades to “religion”
Go count the people murdered in the 20th century in the name of Communism/Atheism, see how that stacks up.