Well, a lot of people out there think of God as someone they can manipulate or re-invent on a whim. If you think that way, you absolutely do not take God seriously. If you think it is "not a big deal" to offend Him, then you are not taking God very seriously. The concept of what it means to offend that all-perfect and all-good God (to whom we owe literally everything), just isn't something they can comprehend. They picture something like a softy, pushover parent, where if the kid gets crocodile tears long enough, the parent melts and just says everything is ok and all is forgiven. And of course, no kid ever really respects that kind of parent.
Problem is, God is not like that, and the slightest offense against Him IS a very big, very serious thing, and He is NOT someone we can just take liberties with and disrespect as we like, and then just brush it under the rug and have everything be well. With God, there is perfect justice. Justice is a concept people can no longer comprehend. God forgives, yes... but that does not diminish the gravity of what we did by sinning against Him, given all of His goodness, all of His perfections, and all that we owe Him. That's serious.
Today, people have ZERO concept of that. They don't care at all. It's nothing to them. So yeah, when you say, to offend that Perfect and all-loving God to ANY degree is a very, very big deal, they just look at you like you're a dope and go, "huh...? Isn't that a little...extreme?" No. Actually it's only by the mercy of God that we don't get the very extreme punishments we deserve for doing something like that. The reality is, yes, offending God AT ALL is really more grave and infinitely more wrong than refusing to offend Him, and letting a proverbial maniac push a button and :heretic: the whole world.
At least that's what I've understood. Of course, if you can find in a real and approved Catechism (from 200 years ago) where it says, "in this situation, if you do this, it's not a sin..." then it wouldn't BE a sin to save the world by doing it. For instance, if in order to protect the world, you pushed the lunatic off a cliff... I'm pretty sure that's called self defense.
But I think the problem here is that people just have lost all sense of Who God is, and what we owe to Him, and of anything like justice. Once you loose sight of those things, then yes, things like this do SOUND "extreme." The more you appreciate the reality of Him, on the other hand, the less extreme (and more obvious) you realize it is to say something like this.