Well one has to question why they would run away when so many others didn't, or tried.
Anders laments that many mages commit suicide.
And I really doubt Aneirin would have been killed, maybe he thought he would being a young and impressionable kid, but clearly that wouldn't have happened and he would have been taken back to the circle and faced reasonable punishment.
The templars attacked him and left him for dead; that's not contested by anyone in the narrative. It's only by sheer luck the Dalish were able to discover him and save his life.
How do I know this? Simple, precedent. Anders escaped the circle 6 times and was still alive on his latest attempt, his 7th.
You're addressing the present leadership without accounting for the fact that different leaders were controlling the Circle when Wynne was younger.
That's when you find him in the keep. He wasn't made tranquil, and he wasn't executed on the spot, he was punished, harsher and harsher because he broke the law repeatedly. In my country we have a 3 strike penalty for repeat offenders so I see the circles and incredibly lenient. Maybe Aneirin's age may have played a factor into it, who knows? Point is, you make it sound a lot worse then the reality.
It's actually fairly horrific, even with Irving and Greagoir in charge.