Here's the thing, in history there's a tendency toward knowledge becoming codified and set in stone, only for a new generation of scholars to take the complete opposite view and argue that point with their own list of evidence. Usually, the answer tends to be somewhere in the middle of the two sides, but not always, sometimes one side is just wrong.

In any case, that debate is happening right now for the Crusades. After being viewed as basically open unprompted Christian aggression and often ridiculed as destructive failures that view is being pushed back upon with some pretty good evidence. Though, for my part I do tend to sit in the "the answer is somewhere in the middle" side of this debate.

What is true is that Muslim forces were pushing into Byzantine territory. And pirates which may have had at least a little backing from Muslim states were making a bit of a mess of things in the Mediterranean. It's worth noting that many of the cities the First Crusade conquered had been Byzantine territory only a few years prior. However, some of the land had been lost centuries prior, like Jerusalem itself. You can make the argument that retaking these lands would create a much stronger buffer state to the Christian East, which in fairness, it did for a while. But you can also argue that the Christians were conquering territory they had lost their claim on centuries before and were now directly attacking.

Now one thing he is a bit right, is that mass killing after a siege was not uncommon, and the extent to the mass killing in Jerusalem is debatable. To go into broad strokes about how warfare was conducted. The attacker starts the siege and tries to outlast the defender. If the defender gives up and accepts some terms, then usually things turned out ok. Some money would exchange hands, maybe some hostages would be taken from the military elite. The details depended on the specific siege. But usually this was the more cordial way of solving the issue. Not always there are examples of sacking taking place after agreements were made, this is often when a leader loses control of their army. The one that comes to mind the most readily is the Siege of Jaffa where Saladin had already an agreement pretty much set up for the Christians to leave the city, only for his troops to completely ignore him and go pillaging. Though Saladin himself then tried to take the loot away from his soldiers as punishment. Which while seemingly very high-minded of him, directly effected how loyal these troops were in the coming battles.

So that's if the city gives up. If a city is assaulted and won through violence, it's very hard to then control the outcome of the battle even if the commanders want to. But there is still a chance, though somewhat slim, that if all the fighting ends at the walls and obvious defensive structures, the sieging commanders may be able to get their men back in order to have a more organized less violent looting. In this situation we can occasionally get strange details about soldiers walking into civilian homes, politely asking for a meal and a trinket and walking back out. It can be done, and even was occasionally during the Crusades.

However, if after the outer walls and obvious defensive structures are breached and then the fighting still continues within the city limits, there is only chaos. There is little to no chance for a massacre not to take place. In the siege of Jerusalem fighting took place within the city limits, in the dark, after the Crusaders were nearly starved for weeks. No matter what, this massacre was going to happen.

However, that leads to the question of how bad of a massacre it was. The Gesta Francorum uses very colorful language to say that the blood of the dead Muslims was up to the crusader's knees. And that by the end of the siege there were no Muslims left living in the city except for those enslaved and the few that surrendered to Raymond and were sent away.

However, once the siege had ended and some semblance of order came about, we see Godfrey de Bouillon who became the ruler of Jerusalem start making edicts on how to appropriately treat the remaining Muslims and Jews within the city. So, they couldn't all have been killed. So the Gesta Francorum is exaggerating at least a little bit. How much? Impossible to say. Madden takes the stance that it probably wasn't that much worse than a usual post-siege pillage. But that's at least in part because his whole stance is re-examining the Crusades without automatically thinking things in the worst possible light, which has been what many historians have been doing for decades. Again from my view, I think it's probably fairer to say that the brutality at Jerusalem was above normal, however, the theory that some have posited that this was a planned near genocidal push by the Christians has little to no bearing in historical fact, especially since we have accounts of at least two of the higher lords leading their armies in the sack attempting to save Muslim lives, Raymond much more successfully than Tancred.

From what I've seen, historians have been mostly receptive of his outlook. I can't speak for everyone, but the man at least does his research.

For obvious reasons the crusades are a problematic topic among both Westerners and Muslims.

Anyone categorizing such complex events in terms of plainly "not as bad" and is being apologetic or accusatory, is anachronistic at best, dishonest at worst, and as such fortunately an outlier, however this also goes for the reverse.

So while often times this indeed applies to the views you mentioned., it depends on what exactly is meant by them.

Certainly the crusades were not an immediate response to Muslim conquest of Jerusalem, which at that point occurred centuries ago. But neither was Muslim expansion "a response" to anything (except weakness of the Roman (and Persian) empires), and Turkish conquest of Asia Minor was recent.

Certainly Alexios I Komnenos was "on the defensive" when he made a request for European mercenaries, which in turn gave Urban II the occasion to make his famous appeal to reclaim the Holy Land (which in turn was not was the Byzantines had in mind), and his narrative certainly was effective.

Certainly Urban II was concerned with strengthening his political position, hoping to mend the Great Schism with the eastern Church and be a the head of a united Christian movement. It's also likely he believed that there was indeed a pressing occasion for such movement.

Did the notion of righteous "defensive" holy war matter to some participants? Certainly.

To all? Certainly not.

That there was much solidarity with the Eastern Roman Empire on part of the crusaders is doubtful, given the tensions and distrust which were not forgotten until the the sack of Constaninople by crusaders themselves. Neither did the Eastern Romans envision a holy war- rather just regular war. And yet for a time there was a cooperation between parties who had little in common except their Christian identity, which the Eastern Romans were definitely attempting to spread towards their Muslim adversaries (with only little success).

Some of the Christian population (notably Armenians) allied with crusaders, and for example it was due to the betrayal of an Armenian that the Crusaders achieved the crucial capture of Antioch.

My foreposter addressed sieges and sackings in medieval times.

Contemporary Muslims themselves didn't necessarily see Crusaders as more evil than other threats - it was disunity and warring in the Muslim world that contributed to the First Crusade's success in the first place, and one major reason for Crusader states lasting as long as they did, were frequent, if temporary, alliances or at least tolerances with Muslims against common enemies.

I think it can be concluded, that the Crusades coincided with the emergence and strengthening of a sense of Christian identity, which in so far as it existed, was indeed at various times under threat from Muslim conquest, but ironically, at the particular time of the first crusades, Muslim identity and expansionism as such experienced a dive in importance.