Things to think about before you call someone "Bad"

[Gallus]

Admittedly, I haven't been following this conversation because I've been playing defense on something i misspoke about, this did catch my eye. I'm just gonna take this line to its logical conclusion. If several different players in different instances, who don't even know each other, all say the same thing, like heal more, as an example, then maybe the "offended" player should heal more in this example.

If you are playing a healer and anything goes wrong, every single person is going to most likely complain about your healing role. But you are still assuming that the person complaining is right. If it happens however, very often, it is likely you are doing something wrong as a healer; the causes, I don't know, it could be because they are new, have low expectations, play the game occasionally or even have some sort of physical/mental handicap, it could be a lot of things. The "they refuse to learn" line is the easiest one to assume because it gives you automatic permission to go wild on whoever you think is doing something wrong.

[Vidu]

If you are playing a healer and anything goes wrong, every single person is going to most likely complain about your healing role. But you are still assuming that the person complaining is right. If it happens however, very often, it is likely you are doing something wrong as a healer; the causes, I don't know, it could be because they are new, have low expectations, play the game occasionally or even have some sort of physical/mental handicap, it could be a lot of things. The "they refuse to learn" line is the easiest one to assume because it gives you automatic permission to go wild on whoever you think is doing something wrong.

You are, again, coming up with a lot of explantions why someone might be struggling - all of them more or less legit, depending on the content ("They're new" isnt legit anymore when you're in The Burn).
Explantions are all fine and dandy, but that still doesnt make it okay for those players to be just outright carried by strangers. When we're talking about group-content, we should expect that everyone in the group is able to contribute to this group-effort in a meanigful way that fits their role. My philosophy is that if someone is struggling with something, that they should disclose that - and by that I dont mean disclosing that they have depression or are playing with their feet, but to inform your party that you might not be able to meet the baseline expectations and ask them if they're okay with that.
Also: if you know that you're struggling for whatever reason, you shouldnt pick a vital role like healer or tank.

I also found that often people dont "refuse to learn", but are not aware that they even need to, thanks to the bad feedback-system the game has.
If a healer isnt healing me enough I can spam clemency on myself, the DPS and in very disturbing cases, the healer to keep us all alive. The healer did a shitty job, regardless of that, we cleared the boss and the healer leaves thinking "Well, I must have done a good job here, no one died!" when in realtiy they did such a poor job that I sacrificed my strongest DPS-skill in order to heal through.

Regardless of your personal circumstances: Its your personal responsibility to go into group-content prepared and able to perform well (enough) within realistic, basic gameplay-expectations. And unless you show that despite your shortcoming you respect your team and the time other people put into this dungeon (for example by leaving a quick "Hey, I'm new to this dungeon/job! Might mess up, but giving my best!"), I dont care about your circumstances. If you just expect me to pick up your slack and show me that you dont care for your team - why should the team care for you?
I shouldnt have to guess which of the 500 possible explantions fit you if you're playing subpar. And I dont want to go around and assume that everyone who havent figured out their rotation at level 70 is mentally challenged or plays only with their nose because they broke both arms and legs.

[Gallus]

within realistic, basic gameplay-expectations.

That line is going to differ significantly from one player to another. Duty Finder content is easy to the point I've seen some lvl 70 dungeons be soloed by a PLD, it shouldn't generally put too much stress or pressure on people, even if there is someone underperforming. There is never going to be a situation where all 4 players are contributing exactly the same.

[Lastelli]

If you are playing a healer and anything goes wrong, every single person is going to most likely complain about your healing role. But you are still assuming that the person complaining is right. If it happens however, very often, it is likely you are doing something wrong as a healer; the causes, I don't know, it could be because they are new, have low expectations, play the game occasionally or even have some sort of physical/mental handicap, it could be a lot of things. The "they refuse to learn" line is the easiest one to assume because it gives you automatic permission to go wild on whoever you think is doing something wrong.

Three observations:

1) I've been playing since 2.2 and I can count the number of times a bad player was called out in pf/df on the fingers of one hand;

2) Never have I witnessed the situation you described, namely the healer being blamed for anything that went wrong;

3) The funniest thing is that even assuming that "toxic players" are a majority in df and pugs and everyone starts shouting at healers whenever something goes wrong...do you know what players are actually able to tell who did his job and who didn't? Yeah, veteran raiders. People that unfairly blame an "innocent" or competent player are usually unskilled players since good players can easily understand what's going on and where the fault lies.