If Good isn't morally upright and correct it is meaningless noise.
That's your opinion.
In the game world Evil people see it totally differently. They're correct, and the Good guys are wrong.
And if you are going to declare other player's interpretations of Good as subjective, while gleefully declaring your own as objective and noting how they will go to Hell for their actions, then what is the point of Alignment?
That is the
point of alignment.
And as DM I get to determine objective reality of the game world. If I tell the Players that a rock falls from the sky, a rock falls from the sky.
The PCs on the other hand are stuck in Cartesian doubt.
And we can get into very precise things where this falls apart. Say for example you sneak up on a cult of demon worshippers and the wizard casts Cloudkill to try and take them out before they can turn and retaliate. CloudKill does poison damage, does poisoning unsuspecting people count as a good act or an evil act?
Depends. Was the act reasonably necessary to protect innocent people from being killed, and was the act proportionate in the circumstances.
That's the only relevant question.
Do you have the bad guys spill the beans on their plans if the players capture them?
Depends on the bad guys.
Do you have prisoners attempt to kill the players in their sleep?
Depends on the prisoners.
Do you make it more and more difficult for them to do the Good thing?
Not intentionally, but doing the Good thing is often hard.
Sure, "being good shouldn't be easy" but we are playing a game. And if being good means getting the snot kicked out of you every time you face an enemy who surrenders, then the players are going to get real sick of it real fast.
If you're a DM that has enemies surrender, just to attempt to murder the PCs in any sort of regularity, you don't deserve to be DMing.
They might try and escape captivity of course. But why on earth would they surrender (to a more powerful force) allow themselves to be disarmed (putting them at an even greater disadvantage) and then attack the same powefull creatures that spared them all over again.
Presumably they've formed the view that the PCs outmatch them when they surrendered. Desiring to survive, most creatures flee when given the opportunity to do so.
A few even remember being saved, and might even pop up later on to return the favor and help the PCs.
You know; like normal living creatures.
And then we are going to get into potential issues of "what is evil"
I covered that in session zero remember.
Evil = harming a creature unless reasonably needed to protect innocent life from harm or in self defence from an imminent threat, when no other option is reasonably open to you.
Dude goes for a weapon; you shoot him dead.
Can I use the suggestion or Dominate Person spells, or is magically robbing a sentient creature of free will Evil?
It's a form of (non lethal) harm. On the lower end. If a creature poses you harm, and you dominate person them to stop them from killing someone, that is in no way an evil act.
If you dominate person a harmless creature, and order it to murder its friends (who also pose no-one any harm) that's an evil act.
Again, see the definition of evil above.
There is so much grey area, it ends up being fairly arbitrary.
Lucky we have a DM isnt it?
If you want to maintain a good alignment, you refrain from harming others unless that harm is in self defence, or the defence of others, is proportionate to the threat, and no other option reasonably presents itself.
If you go around murdering people, raping them or torturing them, you're evil.