Okay, so we'll break it down in increasingly narrow boxes :
1- Hatred of religion in general
Religion has been and continues to be a tool for legitimizing power structures. As such, they're associated with the oppressive systems they've historically upheld. That goes from state structures like monarchy to modes of production like slavery, reaching whithin family dynamics, like patriarchy. Atheism was until recently broadly criminalized throughout the world. Atheism, or at least secularism, is a feature of many ideologies that arose since the late 18th century to uproot these systems, while religions are claimed by ideologies that arose in reaction tooto these. Politically, religion is therefore seen as conservative .
2- Hatred of Abrahamic religions
Some see these religions (Christianity, Islam, Druzism, Judaism) as inherently more oppressive than other, because they see it as a sin to worship gods other than their, and do not permit synchretism like other religions do.
This is a big debate, and it doesn't hold true to the same extent for all Abrahamic religions, but I'm not getting into details on this.
3- Hatred of Christianity
There are religious people who hate christianity for theological reasons, but I'm once again more interested in the political aspect.
Many hate christianity for its role in colonization and association with western imperialism. It's basically the first two points applied to specific circumstances of christianity : How it came to eradicate paganism in Europe and attempted (sometimes continues to attempt) to do the same globally, how, using colonial means it erases cultures. Same can be said of Islam to an extent, and you could say the main difference is that Christianity has been successful more recently and more globally. But then again, while they've had the same attitude towards paganism, Islam has historically tolerated more religious in their states, as long as these were "religions of the book". Tho it's true that the situation has come to be a bit reversed on that point...
4 a- Hatred of Catholicism for theological reasons :
Idk about the details, but part of a reason why the reform happens was that catholicism was seen as drifting to far from true belief and focusing on preserving and enriching itself as an institution. Other accusations include being too close to paganism due to the cult of saints and the continuity with Roman institutions (diocieses that predate the christianisation of the empire, nuns too similar to Vestal virgins...), and the fact that at the time, the mass was always in Latin although most people didn't understand it.
4 b- Hatred of Catholicism : Post-colonial reasons
Point 3 applied to former colonies of France, Spain and Portugal, for which evangelisation was purely Catholic. the Catholic church continues to wield large power there, and some people consider its influence a remnant of colonialism.
However, it is interesting to note that catholicism can be seen the opposite way by people from catholic countries for which a protestantprotestant country is seen as a greater or more recent oppressor (like Ireland or Puerto-Rico).
4 c- Hatred of Catholicism : Structural reasons
Catholiscism has a strict clergy hierarchy, moreso than Protestantism or Islam. Some see this as wrong in itself for philosophical or theological reasons, but even if that is not the case, this means the Catholic Church, more than others, can be perceived as an institution with agency and responsabilities. More than others, they're expected to answer to their past positions and to solve their ongoing issues, something it struggles with as it doesn't want to alienate the more concervative parts of its clergy. Also, this leads me to the next point :
4- d : The pedophilia thing.
This is already very debated in the other comments, so I'm not gonna get in the details, but clearly not a good look.
4- e: Catholics in majority protestant country
Ecumenical debates pass over the heads of most christians, who just go with the form of christianity that is the "default" in their country/region. But what about those raised in families that hold on to their religions where it is a minority?
This point seems important to me because a lot of the debates about Catholiscism I see online are really about catholicism in the USA, where it is a minority. In such countries, catholics will usually be seen as more pious and conservative than other christians, and quite often they'll be, for several reasons :
Principle of selection: A family that is just vaguely christian, upon immigrating, could've converted to better integrate. If they didn't, it's either because their specific belief was important to them personally, or because the community in which they integrated was mostly of other catholics, making it a solidarity factor which in turn increases its importance.
Sense of responsability : If holding a belief is an active choice rather than going with the flow, believers will tend to give it more mind. They may feel they have more of a personal responsability to uphold it. This can culminate in what's known as "siege mentality": People who feel like an important part of their way of life or their belief system is threatened will react with fervor, oftem violence. This is why so many religious or political speech relies on tropes such as "They're trying to steal/destroy/corrupt our thing!"
