Paul W. answered 04/26/19
Dedicated to Achieving Student Success in History, Government, Culture
Your question, needless to say, covers a lot of ground. I suppose that the first thing to say is that YOU can do anything you want. Assuming you live in the U.S. or some other Western nation, you are free to follow whatever religious beliefs you wish - or none at all - so long as you don't harm anyone (I admit, a complicated issue in itself).
From what you say, it sounds like you enjoy the aesthetics of different Faith systems. Without question, among the positive products of religious faith have been wonderful works of art and music. You also seem to enjoy exploring the ideas behind different Faith traditions.
So, about your question, 'Why do people in religions have to be so mean to each other?'
First, it should be noted that history demonstrates that people who practice different religions are perfectly capable of getting along with one another. Moreover, I would argue that peaceful co-existence is far more common than one might imagine, indeed it may well be the norm in human history. Note that this doesn't mean that people practicing different Faiths who live in the same communities and cooperate with one another necessarily love each other!
Nevertheless, history also shows that differences in belief systems can all too easily lead to hostility, including violence. You suggested that it would be more 'fun' if people of different Faiths got along with one another and learned from one another. Personally, I more or less agree with you. But, fundamentally, religion is not about 'fun.' Religions are concerned with THE most serious issues that human being are forced to struggle with, namely, life and death. It is what the followers of different religions believe is at stake that has the potential for producing fear and anger towards anyone who practices a different religion than one's own.
Humans live in an unpredictable, dangerous world. There are all number of things that cause us to suffer and ultimately die. The human subconscious demands a state of safety / security that comes from certainty in an uncertain world. The teachings of religions provide that sought after certainty. Instead of a random, amoral state of existence that is reality, (most) religions teach that someone is actually in control and that, in order to avoid suffering and even death - by experiencing life after death - one need only follow the dictates of one or more supreme beings, as interpreted by their self-appointed representative her on earth.
And here's the problem. An individual derives peace of mind - a satisfied subconscious - through believing in the teachings of a particular Faith, which provides a sense of certainty, an understanding of how the universe works and, therefore, how one can navigate life in order to avoid suffering and achieve a comfortable afterlife. But the very existence of people who are just as certain that their faith system has all the answers can, for some people, call into question the validity of their beliefs and, therefore threaten their precious peace of mind. Again, for some people (fortunately, not the majority of human beings) the only way in which to preserve their precious peace of mind - a subconscious free of uncertainty - is to eliminate in some manner people who believe something different from themselves.