As with something that involved so many changing belligerents over such a long period of time, the answer to this is it depends at what time of the crusades you ask this question and looking at how the Muslim world developed before the Crusades started helps answer why they could possibly hold these views.

Starting from the very beginning the Muslim world wasn't sure what to make of the news that crusaders were coming all the way from western and central Europe. It wasn't their old foes the Byzantines who they did view as something of a religious foe to war against but people that they knew very little about and weren't sure of what exactly was their aim and if they were mercenaries under the Byzantines. The disastrous precursor to the 1st Crusade, People's Crusade didn't lead to any credibility being given to the main crusading body either as it crossed the Bosporus.

The 1st crusade was the greatest success of them partly because of how unexpected it was but also by how politically fractured the Muslim world around the holy land was. The Caliph in Baghdad was no more than a figurehead by this point in time, controlling nothing more than the city and it's surroundings but still holding some spiritual authority. Local power factions from Damascus, Aleppo, Antioch, & the like saw their neighbors as the immediate threats and stepping stones to succeed and bring about a new master to the Levant. The strongest power in the region, Fatimids in Egypt, were Shia and because of it were distrusted by the other regional Muslim powers as much as the invading Latins. The regional infighting left enough space for the crusaders to escape near disaster like at the siege of Antioch where the Crusaders didn't have enough men to completely encircle the city for a siege and a relief army was suppose to be on the way but distrust between the local warlords and treachery by someone in the city gave the crusaders enough time to enter the city and escape being enveloped by two enemy forces.

The 1st crusade had another benefit that helped in it's success in the form of the Byzantines who provided knowledge, supplies, and logistics along the way. Eventually that went away as it became clear to both them and the Muslims that the Crusaders weren't there to help the Byzantine Emperor reclaim lost land but to establish their own petty kingdoms. The Byzantines were already weary of how exactly the 1st Crusade had formed and after it they would not welcome another crusade or may even be hostile to them.

That of course was just one side of the coin in how fractured the political environment could be in the holy land. The fortunes of survival of the Crusader States depended on how fractured their Muslim neighbors were. The Crusader were seen as invaders but there were would be over the course of the time period of its establishment after the 1st the crusade to the fall of Acre that the Muslims would treat them either as a political ally or adversary or a religious target for Jihad.

The 6th Crusade and the Baron's Crusade that would follow it were both successful in recovering land but not because of massive victories but because of diplomacy and the regional Muslim powers being preoccupied in other matters where they viewed giving up land as a necessity to focus on a more pressing internal concerns. The former, a rebellion in Egypt and the latter, a war between brothers. A good example of the gray area that the Crusaders States found themselves in was in the lead up to the Battle of La Forbie, where they, along with the military orders, fought alongside their Muslim neighbors against a growing Muslim power that they all feared. This battle would be a crippling defeat that the Crusading forces would never recover from.

Two of the biggest threats to emerge over Crusading States time period to their survival were when all the surrounding Muslim lands were united under a singular rulers. The first time being under Saladin and the second time being under Baibars. The 1st time crusaders states managed to hang on to survival and the 2nd time they would not. The Crusader States also played a large part in the rise to power of both Saladin and Baibars in that they became beacons for both rulers to rally Muslims around as a common enemy.

Saladin main goal after his rise to power was eventually conquering all the neighboring Muslim lands but after having a near death experience and some aggressive actions by the Crusaders, he felt it was a sign from god that had err'd in his ways and should actually be not fighting his brother of the faith but waging Jihad against the Latins. This would be what lead to the fall of almost all the mainland positions of the Crusader States and the call for the 3rd Crusade. Of course conquering the holy land served another purpose for Saladin which was as a legitimizing force for not only for himself but for his just established Ayyubid Dynasty.

During the Mongol invasions, the Crusader States had sided against the Mamluk Sultanate ruled by Baibars expecting its defeat like all others up to this point but Baibars would emerge victorious. The period of mongol expansion would end in the middle-east and Baibars would start a renewed Jihad to wipe out all those who had sided with the Mongols. This Jihad would also serve in establishing his new dynasty with the prestige of not only as the man who defeated the Mongols who had already brought so much ruin to Muslim lands but also who finally retook the Holy Lands. Crusade was called again but by this point the crusading spirit to go to the middle east back in Europe had ended. The fall of Acre in 1291 would mark the end of Crusader States presence on the mainland while the Kingdom of Cyprus would go on till it would get swallowed up by a rising Ottoman Empire centuries later.

The way Muslims viewed Crusaders forces as the history of their interactions shows is complicated. Part of it is that you still had many Christians such as Jacobites, Nestorians, and the like that were left behind by early schisms and isolated by the Arab conquest that lived in Muslim ruled lands otherwise known as Dhimmi. Muslims saw all Christians as people of the book and not heretical like they saw Fatimid Egypt. Where you do see religion play a role in how they combat the crusaders was that it could serve as a rally cry since Jihad meant the conquest of land that wasn't ruled by Muslims otherwise known as Dar al-Harb to land ruled by Muslims otherwise known Dar al-Islam. This had served as part of religious life for Muslims during the time of a strong Caliphate neighbored a weaker Byzantine. It didn't always have to be conquest either but just raids that would fulfill the pilgrimage that these Muslims had come to the northern frontier to do. You do see slaughter play a larger rule though as the Latin Crusaders fought against the Muslim forces in part because atrocities committed by the conquering Crusaders on defeated or captured towns would be payed in kind but if you go by the Arab Conquests of both Byzantine and Persian lands, it wasn't a part of the process. Forced conversions weren't a focus either of Jihad since Dhimmi real penalty would be having to pay the extra tax of the Jizya and Caliphs built separate forts to garrison their soldiers so they wouldn't need to intermingle with the Dhimmi when the Arab Conquests were ongoing. This helps shape a view that the Muslim view of the crusaders was that they could fulfill a religious role by fighting them but they didn't view just being Christian as being something worthy of death since there plenty Dhimmi in their own lands. What made things so contentious was that atrocities that were committed by Latins, who viewed Muslims as something foreign, were returned in kind but non-Latin Christians and Jews were usually spared these retaliations.

Some books that offer more detail into this subject:

The Crusades Through Arab Eyes by Amin Maalouf

A book that gathers both Muslim contemporary chronicles and participant accounts of the crusades.

The Prophet and the Age of the Caliphates by Hugh Kennedy

This book focuses on what came before, helps explain what comes after.

A little further a field but before the Battle of Manzikert the local Christian power that the Muslims had to contend with were the Byzantines and their interactions inform how Muslims in this region viewed Jihad and frontiers with Christian neighbors, here is some stuff adds further details in this particular direction:

Arab-Byzantine relations in early Islamic times edited by Michael Bonner

Deals with the relationship between Byzantine empire against a ascending Caliphate one part where after conquering and the beginning of deep raids into Anatolia.

Streams of Gold, Rivers of Blood by Anthony Kaldellis

This is through the Byzantine lens but does deal with a critical time period of 955 to 1st Crusade that coincides with the fracturing of the Caliphate, the rise Seljuk Turks, and as well the establishment of the crusader states.

edited for spelling and cleaning up sentences