A Buddhist extremist cultivates a state of calmness and kindness, and cultivates the ability to keep her or his attention in in the present moment and not in the past, the future, or lost in thought.
These are Buddhist extremists.
If Zen devotees work hard, most of them will achieve a state of abiding inner peace and a profound and lasting feeling of kindness toward others.
Islamic extremists try to practice Islam in its pure form. They try to do things the way Muhammad did them, and they try to follow Muhammad's teachings. They live austere lives devoted to jihad. They don't sit around contemplating their navels. They prove their devotion with action. They try to make the law of Allah the supreme law of the world. They devote their lives to fulfilling the political goal of Islam, just as Muhammad dedicated his life to it, and just as Muhammad taught his followers to do.
An Islamic extremist cultivates hatred toward non-Muslims and works toward the day when all non-Muslims are either subjugated as dhimmis, converted to Islam, or dead.
These are Islamic extremists.
If Islamic devotees work hard, most of them will find themselves in some form of warfare with non-Muslims and ideally will be killed fighting in the way of Allah.
Are all extremists dangerous? Are all ideologies the same? Would it matter to you what kind of extremist you had as your neighbor? Would it matter to you what kind of extremist your children chose as close friends or heroes? Would it matter to you what kind of extremist your country allowed to immigrate to your country?
Can we please stop using the word "extremist" when we mean "devout Muslim" or "dedicated follower of Islamic doctrine?"