I don't particularly care what someone who murders a bunch of people "thinks they're doing." Perhaps that makes me intolerant.
The point is that this "Paradox of tolerance" meme is invokable to escalate the aggression of any viewpoint - a paradox in the common (not not A -> A) logic system means anything can be proved.
We don't have a problem with the murderer and his enablers because they were intolerant, but because they committed and encouraged murder.
Can't really discourage the next one at this point unfortunately. People are going to do things like this, they have been for a while and will continue to do so in the future. These ideological extremists will be arguing back and forth forever.
All that said, there is the completely separate question of what our counter-terrorism infrastructure should be doing to identify and neutralize these threats. But that's not so much "discouraging" the next attack as it is attempting to keep the number of attacks to a minimum. Because I can pretty much guarantee, the counter-terrorism guys in NZ, or most other western nations, are not gonna be visiting suspected terrorists for the purposes of having a conversation in the hopes of "discouraging" them.
This is, of course, in _addition_ to other counter-terrorism infrastructure like intelligence gathering, monitoring, security, response, etc. Many of those more well-known systems are intended to identify bad actors and prevent them from doing bad things; understanding bad actors can lead to more methods to prevent regular people from becoming bad actors in the first place.
Here's the thing, people been doing this for literally MILLENNIA now, there is no way to identify everything that will drive extremists to kill. That's the essential problem with that philosophy. Even where you are able to identify the core issue, say for instance the religious divisions in the MidEast region, there is really no way to address the core problem. It's just far easier to engage the extremists, than it is to convince Christians, or Jews, or Muslims, to NOT be Christians, or Jews, or Muslims.
Extremists will just never be placated, and we shouldn't put too much effort into trying, because that gets people killed as well.