Ho boy... there's three parts to this question that we need to answer one at a time and in order because there's a vastly incorrect premise here.

• Did East Asian religions have a much more peaceful exisence?

• Are East Asian religions inherently more pacific? If so, why?

• Why do we perceive East Asians to be pacific and Abrahamic ones to be violent?

So let's start from the top:

Did East Asian religions have a much more peaceful exisence?

And the answer to this is a resounding absolutely not.

As you can see from my flair, I specialize in Buddhism. A few weeks ago I answered a question regarding wars based on Buddhist belief. And shortly after that there was a question regarding the Slovenian Philosopher Zizek and his perception on Buddhism (somewhat related). Feel free to read those and get back.

The long and the short of it is that eastern religions, with a laser focus on Buddhist traditions there in that thread, have been no less susceptible to war, genocide, and explosions of violence than in Abrahamic traditions.

Gandhi's deep commitment to pacifism hasn't been universal. He was murdered by a radical Hindu. Of which there were many who advocated violent resistance against both the British and the Indian Muslims. The most radical of them, the RSS, formed the basis of various Right-wing Hindutva based political parties, leading up to the modern BJP (but that's sure as hell not a can of worms I'm interested in opening up right now...).

We don't even need to look at the modern day to look at violent Hindus. In the brief window between the Mughal Collapse and the British Raj, the Marathas took control of India, and were not shy about their use of violence against their enemies. Throughout Indian history, in fact, there was a whole segment of life (referred to as yoga) that specifically referenced wandering men in the "war market" of India. This market seems to have taken many many changes over the course of Indian history, culminating in the British roundly defeating, dissolving, and coopting these organizations (and the market entirely) when they conquered India.

In China, Confucianism and Taoism, sure, seldom advocated violence in any systematic manner (as far as I can tell), but Confucianism, which placed the Emperor at the center of a properly ordered world-society, could be argued to imply a violent correction of world order. Which I would argue, is exactly how dynastic China operated. When a Dynasty which had lost the Mandate of Heaven was being violently upset, a new Dynasty which had obtained the Mandate (or claimed to) violently established their order.

Buddhism didn't seem to entirely temper the Tibetans (or Koreans, or Japanese) imperial ambitions. It's arguable how Buddhist the Tibetan Empire became, but even today the Three Ancestral Dharma Kings, whose deeds are mostly remembered for the first establishment of the Dharma in Tibet, are also known (much more generally) for their military accomplishments.

The Korean Silla and Goryeo periods are characterized by Buddhism, but are also known for dramatic abuses of power (including conquest and violence against other Buddhist powers). Japan's history, especially as regards Buddhism, is far too long in this thread, but Zen theology was used to justify wars against the Allied powers in World War II, straight up excusing soldiers for killing living beings, which of course, Buddhadharma is strictly against. (See above threads for better explanations).

Are East Asian religions inherently more pacific? If so, why?

I would argue yes, but only marginally so. Buddhism, for example, places a much clearer prohibition against killing in the Five Precepts. This differs from the Ten Commandments' commandment against killing in two ways.

First, as far as I'm aware, the commandment is usually interpreted as to refer specifically to 'murder' but excludes the more general term 'killing.' This is important, because while a Buddhist shouldn't murder, he also isn't supposed to slaughter animals, slap a mosquito, or kill in defense. All of which (and I'm not a Hebraicist) the Ten Commandments traditionally seem to allow.

Second, the Ten Commandments are commandments. They are orders distributed from a deity on high. The Five Precepts are vows that must be taken. Since God is universal (and yes, I'm aware that I'm paraphrasing and generalizing Abrahamic religions) his Commandments apply to all of humanity at all times. If we excluded all Christian soldiers fighting for the defense of their nation, we would be condemning a lot of faithful Marines, noble Crusaders, and good Minutemen to fiery hell.

This points to the difference in soteriology of Abrahamic and Dharmic religions. Followers of the Abrahamic God want to obtain some kind of union with him. The goal of Christianity, Islam, and Judaism, is to some degree to achieve an afterlife with God, and not apart from Him.

The goal of Dharmic religions is to achieve liberation from the cycle of suffering and rebirth.

The goal of Sinic religions (Taoism and Confucianism) is to achieve harmony and balance.

These all sound like they might be synonyms for each other, depending on how you want to define liberation, balance, and God. But they are fundamentally different. In Buddhism, one doesn't try to align their life with a deity's commands, but is trying to escape Samsara, which is the cycle of rebirth that leads one back to suffering again and again. Obtaining liberation from this is achieved by (usually) becoming a renunciate (monk or nun) and living a life between the extremes (called the Middle Path).

Going on a crusade would be ludicrously antithetical to this goal. Those Buddhists which engage in religiously motivated violence, usually do so by using Buddhism and Buddhist history as an identity marker, not as an odd theological reinterpretation of Buddhist teachings themselves (again, a vast generalization). Warriors who subscribe to Buddhism are... well, very common in Buddhist history, but the rules of a monastery are much stricter. Taking human life is one of the two breaches of Monastic conduct which can get a monk expelled from the Monastery, and one of the first acts of an Anti-Buddhist sovereign to destroy the Sangha is to force monks into the army, ending the possibility of the Sangha reforming.

• Why do we perceive East Asians to be pacific and Abrahamic ones to be violent?

I think the simple answer to this here is education. Americans learn mainly American history with a bit of Europe thrown in during the Age of Exploration (i.e. the Western "conception" of the idea of America) and then disappears until the World and Cold Wars. In Europe, obviously, they have a different perspective, learning mostly their own history and a bit of American and African history as necessary to the telling of their own national, well, let's call it, "epic."

This leads to a much much larger and longer conversation about the history of public education and history as propaganda, but in the terms of eastern religions, American and European histories don't have a long history with abuses or violence with eastern religions (the key word here is "long"). The equivalent would be classrooms in China, Thailand, and Bhutan asking why eastern religions have violent traditions but Catholic monks seem so peaceful, ergo, why are Christianity and by extension, Abrahamic religions so peaceful? Of course, Western history students with a long history of abuse from Christianity, both systematic and otherwise, know better.

There's other smaller reasons. I.e. Buddhism's and Hinduism's own image of themselves is as "religions of peace" even if they don't refer to themselves outwardly as such. Part of this is the emphasis, see above, on the Precepts which prohibit the taking of any life for monks (though see other threads I have written in on when those Precepts have clearly been bended or broken with few or no intra-religious consequences). The Dalai Lama XIV, for example, presents himself as a "simple monk" whose outward goals have been world peace, a self-determined Tibet (though not explicity independent, in fact he has written extensively on the importance of the Middle Way Approach), and the freedom of religion around the world. He, naturally, has come under fire, particularly from the Chinese Government for being a "splittist" and the religious minority of Dorje Shugden practitioners (a can of worms we don't need to get into here). Be that as it may, the Dalai Lama, in his promotion of a peaceful resistance against the Chinese takeover of Tibet, was awarded the Nobel Peace Prize in 1989. Thich Nhat Hanh is another very popular Zen Master who has been, arguable, the world's foremost promoter of Engaged Buddhism and World Peace, even nominated for the Nobel Prize in 1964 (no prize was awarded that year). Hindu yogis, though I'm not sure any have reached the high profiles of either His Holiness or the Venerable Thay, often times promote themselves in the same manner, or at least as animals of the same stripe - sitting by the Ganges and meditating upon extreme austerities, or singing Vedic hymns while practicing modernized yogic forms - but there's a long list of Yogis and Buddhist monks who actively flaunt or promote violence, particularly in recent times against Muslims, though we're quickly ascending into current events and out of history, but it's not like we can't find plenty of examples of Muslim/Buddhist-Hindu conflict pre 1999.