Why are Christians often really judgemental?

Religions promote intolerance, even when they claim not to. When people form groups, they tend to feel compassion for those in the group and enmity for those outside of it. Religion exaggerates that difference by promoting the idea that they have some kind of absolute knowledge about truth. Religions tend to use emotional arguments to keep members. They tend to portray themselves as the only "saved" and therefore all others as evil. Because they aren't really based on reality, they tend to discourage anybody from listening to those outside of their religion. This tends to breed intolerance. The ones that preach love and compassion tend to do so only to the point they think they can convert someone to their religion. Otherwise, the outsider is rejected and communication is heavily discouraged.