Christian Teacher writes letter to school newspaper saying "Gays deserve to die"

[Zenith]

[NOTE: subject was "But still, there are a multitude of them who, I believe, don't join ISIS for the simple reason that ISIS is an Islamic organization, and not a Christian one."]

I think it depends on where you read from the Bible. Or if you take greater inspiration from books written by "saints" (not your case, you're a protestant), or on what the priest / pastor says. [1] 

Anyway, I was expecting the "they're not true Christians" answer. But for the whole world a Christian is one who considers himself Christian. If one
were to take every man who considers himself Christian and make a list of who or what makes one a "not a true Christian", and then add those lists
together, then every group of Christians would exclude all the others, so in the end all Christians would end up as "not true Christians". [2]

And if one who considers himself Christian tried to impose his religion on everyone else, then every non-Christian would consider him Christian. Same as we call Muslims both those who act nicely and those who are pieces of shit. [3]

1. It's your premise...where does the Bible give any indications that Christians should kill people they don't agree with?

Where in ANY religious holy book of ANY religion does it say "kill innocent people"? Nowhere.

The way they do it is by defining what makes one "innocent" or "wicked". Christians who, given enough incentive, would go out and kill disbelievers, would not go to kill people on the basis of "Well, I don't like them. That's all!" --- no, the disbelievers would be like DEVILS, evil, without morals, diabolical, people who would anyway be going to HELL for their wicked sins, people who continuously and obstinately fight against the Church and against anything that is holy --- the victims would be those whose sin against the Holy Spirit will never be forgiven. They wouldn't be calling themselves "aggressors" - they would simply be doing what is needed, DEFENDING Christianity and honest moral people against a grave pestilence!

2. It does not matter what some people think or call themselves. There is a definition of being a Christian found in the NT. If a church adds to that, then you could have a further distinction: Catholic, Baptist, Coptic...whatever. Because these additional layers are sometimes at odds with each other, then obviously some or all of the components of these additional layers are wrong. Whatever the differences, if a group strays from the basic definition in the NT, they would cease being Christians--because words have meaning. 

I posted this a couple of months ago:

What Christians are called to be (all based in the NT):

1. Has an undiluted devotion to Jesus.
2. Pursues a biblically informed view of the world.
3. Is intentional and disciplined in seeking God's direction.
4. Worships, and with a spirit of continuous repentance.
5. Builds healthy human relationships.
6. Knows how to engage the larger world.
7. Senses a personal "call" and unique competencies.
8. Is merciful and generous to those who are weaker.
9. Appreciates that suffering is part of faithfulness to Jesus.
10. Is eager and ready to express the content of his faith.
11. Overflows with thankfulness.
12. Has a passion for reconciliation.

  List from http://www.christianitytoday.com/pastors...stian.html

Further on this topic, you have to distinguish between people who identify as cultural or nominal Christians. These are people who don't follow the NT, so don't meet the definition, but think it is necessary to distinguish themselves from other religions or from atheism without any real (or at least a superficial) epistemological commitment. 

That's EXTREMELY subjective. Go out of your community, to other people who consider themselves Christians, ask the same question, and from the NT they would give you different answers. Not to mention that sentences like "Has an undiluted devotion to Jesus." can easily be interpreted any way one desires.

3. If someone tried to impose Christianity on everyone else, that would be decidedly unchristian--so what are they really imposing?

He would be imposing Christianity. That's how most of Europe and USA became Christian. If not for that "imposing" then you wouldn't have grown in a Christian-majority country right now (as I am suspecting you do).

We identify Muslims as Muslims because the definition of a Muslim is a follower of the teachings of Mohammad (the Koran). Since the Koran can be reasonable interpreted in a conflicting ways, the definition remains pretty broad. In contrast, the NT is no ambiguous.

That's you, a disbeliever of Islam, talking about Islam. What makes you think that a disbeliever of Christianity wouldn't say the same about Christianity, and even for good reason?