Paladin and rogue in the same group - what's allowed and what's not?

[Mnemnosyne]

Further justification of the lingering evil rule is a little trickier to cite since there is no single source that expands upon it. However, several sources that deal with the outer planes have said that whole worlds and even layers of different outer planes have migrated to one of the other planes when enough of a particular alignment was present on those planes. The lowest layer of mechanus, for example, used to be part of arcadia according to MoTP, and DoTU says that Lloth has dragged whole worlds into the demon web pits by gaining a strong enough following on those worlds.

The fact that a fiend -can- cause a lingering evil with its presence indicates that there is something about its presence that has a corrupting influence on the world its on. Even if the fiend is not actively doing evil, and does not linger long enough to cause a lasting evil in one place, his presence is still bringing that world, whether it's a material plane world or not, closer to being drawn into one of the lower planes. Simply by being there, he's making it easier for others of his kind, who have darker minds, to remove the influence of good and destroy or conquer that world.

As for fiends on the lower planes, if they are good then they're disrupting that plane as well, to leave would cause harm to another plane, while to remain would mean harm to their home realm. Destroying a good fiend on a lower plane is a mercy, and necessary to keep that fiend from losing hope and returning to evil.

If we're going to discuss these utterly ridiculous rules in the first place, we have to stick by the exact letter of the rule, and the rule says evil outsiders cause lingering evil only on the Material plane. By the way, it says 'evil outsiders'. It doesn't say things with the [Evil] subtype. Any outsider with an evil alignment, regardless of its subtype, plane of origin, or whether it acts on its evil alignment, apparently causes lingering evil, according to that rule. And it makes no exceptions for outsiders of the native subtype. If we're using that rule as an excuse for murder, then it means that murdering every single outsider of any subtype that pings on detect evil and is on the prime material plane for a 'long-term', whatever that is, is justified, including tieflings, genasi, etc.

The rest of the stuff mentioned here has no rules by which to judge it on, and therefore can't come into a nitpicky discussion based solely on a stupid set of rules. There are no actual rules for sections of one plane sliding into another - it happens, but there are no rules on it, therefore there's no way to actually account for it in such a nitpicky discussion. It exists purely in the realm of DM fiat.

I wanted to add to the two justifications "it is likely to go on to harm innocents(or regular folk if you don't believe in innocents)."

In the case of a red dragon, while it may not be burning the nearby village to the ground right now, there's no guarantee the good guys will be able to stop it when it is, if they are even in the area. While ideally it could be brought before a court the moment it commits a crime, D&D adventures tend to take place in the wild, beyond the long arm of the law itself. Out here on the raggedy edge, leaving a likely threat alive because you got to it before it got to you is Lawful Stupid.

By this token, I would not be averse to letting a paladin character go so far as to take part in an effort of a kingdom to kill all chromatic dragons in a nearby range of mountains on the justification of several villages at the base of the mountains being destroyed in recent months. Genocide? Maybe. I'd certainly be a little more queasy about it if the paladin were doing it to avenge his family, or because he hates dragons, or for renown. But if his motivation is to defend the innocent lives of his nation or to obey his just king, then he's got the green light from me.

The very passage those justifications come from say plainly, that killing an "always evil" creature just because it's there or because you want its treasure, while they certainly aren't good acts, are not generally considered evil. It only becomces a question of whether or not the act is evil, when the creature in question is definitely of a good alignment.

This is only given with the justification of 'because it still stops the creature's predations on the innocent.' If the creature has made no such predations, there is nothing to stop, therefore it wasn't justified and it is murder. Just because you think someone is likely to cause harm doesn't make it true, and if it's not true, then the murder is not justified and therefore evil. So, you could be trying to stop someone from harming others, but you're acting on wrong information. Still murder, just as if I overhear you plotting to kill someone, murder you to prevent it, and it turns out you were talking about doing something in a game, I still committed murder.