This is simply false. We haven't seen any Dalish clans capture and abuse the PC and their party on sight before and then consider just executing them outright.
No, it isn't. We see every single Dalish clan we meet do something similar.
The opening of the DA:O Dalish Origin is about possibility of brutally executing two humans who you and Tamlen had the misfortune of running across.
When you first meet the Dalish in DA:O (e.g. as a CE or human), they threaten you with immediate execution if you do not turn back, relenting only if you either force your way to Zathrian (where the game forces you to reveal you are a GW) or you reveal you are a GW directly.
When you meet the Sabrae clan in DA2, you are again threatened to turn back, until you mention that you're running errands.
There is also the codex entry that Father Gentivi has about his encounter with the Dalish, which is on the same spectrum,
This clan was more directly violent, whereas the other clans we see are more threatening, but the attitude is the same.
Keeper Thelhen says of the city elves, "The poor creatures in the alienages you think of as elves are but poor cousins, lost to use forever. Some clans might accept a few of them to strengthen their numbers or out of a misguided sense of pity, but they are not our people" and "If you and Gaspard slew each other, and the war killed every human in Orlais, and burned every alienage to the ground... then we would be willing to return to Halamshiral."
That is a sentiment we haven't seen from any Dalish Elves before, even ones hostile to City Elves (which not all of them are such as Marethari and Paivel). Velanna has no respect for them because she believes they are spineless and have forgotten who they are. She gives them an elven annulment so they remember. That's a very different sentiment. And Keeper Thelhen explicitly other Dalish clans taking in other elves, the very thing the clans we've seen do.
Now this is the part where you are completely wrong, because the DA:O codex is clear that this is a fundamental Dalish belief. From their own city elf codex:
"We tell the children that the elvhen are strong, that we are a proud people, but they hear of these city elves who choose to toil under the humans' heavy hand. How do we teach them pride when they know there are others who would allow themselves to be trampled into the dust? So we tell them that these city elves are to be pitied, that they have given up on their people, given up their heritage. We tell them that some people are so used to being controlled that, when freed, they know not what to do with themselves. They are weak and afraid--afraid of the unfamiliar, afraid of our life of wandering. Above all, they are afraid even to hope that one day we may have a home of our own."
This is exactly what Thelhen says: that the CEs are pitied, that they are not "evlhen". That is the first part of your quote, the view being fundamental to what the Dalish believe.
Now as for the second part, about the humans all being exterminated so that the Dalish can take over the Dales. That's also a belief that's lifted right out of the DA:O codex:
"In time, the human empires will crumble. We have seen it happen countless times. Until then, we wait, we keep to the wild border lands, we raise halla and build aravels and present a moving target to the humans around us. We try to keep hold of the old ways, to relearn what was forgotten ... For when the human kingdoms are gone, we must be ready to teach the others what it means to be elves."
The belief that the humans will collapse on themselves, kill each other, and allow the Dalish to move in is again part of their central belief as a people. The language isn't as inflamatory as the one that Thelhen uses, but it's the exact same belief.
Believing that City Elves are not "their people" doesn't mean the same thing as "I wish you all would die." And even that is not a universal attitude amongst the Dalish. Merrill even says she considers the plight of the City Elves in the Alienage to be the plight of "our people" in one of her conversation with Fenris. And in another she tells Fenris that the Tevinter slaves should go to the Dalish for help. And in the case of Alarith in the Denerim aleiange, that's basically exactly what happened. And Joshmael tells Tallis "The elves are all one people" in Dragon Age: Redemption.
Thelhen doesn't say that he wishes all of the CEs would die - he says that he wishes all of the humans would die.
And as much as you give examples of specific (and kind) Dalish characters from the games, we have clear codex entries about what the general beliefs of the clains are in the setting.
Sorry, but you're wrong.