Because it is much easier for people to universally accept a system where good or neutral deeds are expected by default, and misdeeds are punished.
It is very difficult to construct an alternative system that humans could internalise. Where would you draw the line? What about saving 50 people, and then killing 49? Should they cancel each other, too?
If a police sniper shoots a mass shooter in the middle of their mass shooting, that's a hero. Not a villain.
We have to do the best we can to be kind and minimise suffering, while understanding that there will inevitably be a diversity of judgements on moral matters. And if those moral judgements have real-world effects, there will be moral judgements about that too.
The lack of moral universality is how it is, not a failure. And it never ends: there are no right answers, although there might very well be wrong ones. Its up to us.
I don't mean this as a "gotcha", but as a reminder that morality is a human invention, and different humans will take different moral stances on things.
Only if they were linked - you blew up a plane that was about to be flown into a building for example.
That's completely different from one day taking over a plane and landing it safely because the pilot was out of action, and the next day shooting down a plane for fun.
You can't save up to murder your wife by giving to the homeless.
But even if it is, in fact, a thing - it's clearly not backed by "one particular moral judgement", as it is commonly portrayed. Lots of people face disapproval and punishment for a diversity of chosen moral stance, including people who could be categorised as "liberal" and who are typically considered to be those doing the "cancelling".
Supporers of the abolition of slavery or apartheid, or of human rights for minority communities, were for many years "cancelled" in the US, and in Europe, for example. Today, in the US, supporters of social equality and diversity are being "cancelled".
So I suspect that "cancel culture" is what you get when one moral/political group (of any persuasion) only sees part of the bigger picture, and uses that to manufacture a grievance.
Now, the worst and slimiest amoung us are crawling up on the cross and weeping and gnashing their teeth because people won't buy their book or watch their movie. It's almost always the most powerful who claim to be 'cancelled'.
Calling out assholes is a good and useful function and we should continue to do it.
That's a bad example (because all 99 will die anyway if you don't do something, so you're not really killing 49 to save 50), but ignoring that, I don't think you can trivially answer such questions. They have been discussed by many philosophers for the last few thousands of years and we don't seem to have a common agreement about ethics and morality.
Would you change your answer if the building was a prison for 50 child abusers, and the plane carried 48 newborn babies (plus the pilot)? Why? A human is a human, right?
And you are also missing the point of the comment - the key thing is the principal of least harm only applies if the things are directly linked.
I suspect you'd find it hard to find a philosopher over the last few thousands of years who thought that the concept of saving up societal credit so you can kill you spouse is somehow a valid one.