New Zealander jailed for posting a picture of a certain religious leader

[naut]

An icchantika is, depending on translation, often considered a person who actively hinders people's achievement of enlightenment. So your dharma towards them would probably not lead to unfavourable karma, because your dharma would benefit other people's dharma and potential attainment of enlightenment. Also keep in mind with certain sutras the moral polarity of your dharma is quite fluid, and not hard and fast codes that must be ascribed to in their totality (e.g. Hammurabi's Code of Laws, etc).

An analogy could be putting a murderer to the firing squad. You transgress them because the sum total of their transgressions would be larger.

P.S: I am not pro-killing people, I'm just trying to put forward a potential explanation for that sutra.

[Husar]

An icchantika is, depending on translation, often considered a person who actively hinders people's achievement of enlightenment. So your dharma towards them would probably not lead to unfavourable karma, because your dharma would benefit other people's dharma and potential attainment of enlightenment. Also keep in mind with certain sutras the moral polarity of your dharma is quite fluid, and not hard and fast codes that must be ascribed to in their totality (e.g. Hammurabi's Code of Laws, etc).

An analogy could be putting a murderer to the firing squad. You transgress them because the sum total of their transgressions would be larger.

P.S: I am not pro-killing people, I'm just trying to put forward a potential explanation for that sutra.

Then that would depend a lot on who exactly is defined as hindering other people's enlightenment. So probably all christians, muslims and possibly atheists as well if they preach non-buddhism to the people.
And then there is the next part:

In addition to issues of faith and unbelief, the Buddhist tradition offered sophistic justifications for killing and war:

[H]ow can one kill another person when…all is emptiness? The man who kills with full knowledge of the facts kills no one because he realizes that all is but illusion, himself as well as the other person. He can kill, because he does not actually kill anyone. One cannot kill emptiness, nor destroy the wind. [25]

Furthermore, killing is sinful because of the evil it creates inside the killer’s mind. But, a true yoga master can train his mind to be “empty” even while he kills. If the killer has “vacuity” of thought, then the murder “did not undermine the essential purity of his mind” and then there is nothing wrong with it. [26] In other words, killing can be excused if it is done by the right person, especially a “dharma-protecting king”.

[Viking]

If the killer has “vacuity” of thought, then the murder “did not undermine the essential purity of his mind” and then there is nothing wrong with it.

In other words, airheads can safely kill anybody they feel like.

[naut]

[H]ow can one kill another person when…all is emptiness? The man who kills with full knowledge of the facts kills no one because he realizes that all is but illusion, himself as well as the other person. He can kill, because he does not actually kill anyone. One cannot kill emptiness, nor destroy the wind.

That's the metaphysics at the core of buddhism. That, in a sense, the universe is an eternal soul, and every person just an iteration within it. So if you kill someone, you don't really kill anything because "a person" doesn't really exist. They're just the incarnation of what the universe is doing at that location and that point in time. What really exists, i.e. the totality of all things/the universe, will still exist and never dies.

It also ties into the idea of spontaneous doing, of having "natural mind", unhindered by concepts:

[T]he fire in the bush burns the mountain;
the hurricane breaks trees;
the collapsing cliff crushes wild animals to death;
the running mountain stream drowns insects.
If a man can make his mind similar,
then, meeting a man,
he may kill him all the same.

- Chan Sutras