You are going to have to provide citations for your point about Dalish Clans regularly looting and killing, because considering the nature of Human treatment of Elves, they could possibly fall under self-defense, depending on the nature of the "traveling" humans and where they were traveling, and dozens of other considerations.
As to the point about the Dales falling, the idea was they were separating in order to re-establish their culture. Neutrality actually has advantages in certain conflicts, depending on the exact situation, it's not always so simple. At worst, you could say it was a strategic mistake, not some kind of abandonment, and considering the treatment of the Orlesians of the Elves during that time it's kind of easy to imagine why they might of been hesitant, why aid those who were just as likely to kill them?
The part about blood rituals and stuff technically fall under rumors, derived from human sources no less.
1) Play Origins and talk to one of the Elven hunters after finishing Nature of the Beast. The hunter, a DALISH, will bluntly admit to you some clans actively hunt down, kill and loot caravans of travelling human merchants.
2) Play Jaws of Hakkon again and read the Codexes. The Dalish broke their promise to help Orlais against the Blight. The elven armies watched as Montsimmard was invaded by darkspawn and burned to the ground. Had Ameridan survived while solving the problem posed by the Avvar and Hakkon, he would've united the Dalish and led them to march alongside humans.
Ameridan, the only Dalish rational and clear-minded enough to understand it's stupid to let humans die in a Blight because of racism. What a pity many other elves are not as evolved as he was and prefer to blame humans for all their misfortunes instead of being clever and brave like Ameridan.
3) Those are not rumors. Play Inquisition again and head for the Temple of Mythal with Solas in your party. Click on the three Mosaics with depictions of the Elven gods and Solas will tell you of the blood rituals performed by Falon'Din.
So it's not a human source, as you can see.
Two elves openly admitting the flaws and crimes commited by the elven people. One admits Arlathan was already a decadent civilization by the time elves like Falon'Din were mistakenly considered gods. The other admits there are Dalish clans - in PLURAL - who are no more than bandits preying on innocent people.
Elves are as mediocre as humans, dwarves and any other race. The fact they didn't lift a finger to save innocents from a darkspawn invasion shows their true colours. Not even their immortality was inherent to their race. Otherwise, they would still be immortal after the Veil was created.
The way I see it as a player, they brought their fate upon themselves and will continue to suffer by their own hands until they learn they are arrogant, violent and wrong in their assumptions about their past, their identity as a people and learn to improve their relationship with the rest of the world.