Wrong. That's an assumption based on an unfounded subjective absolution.
Except that the virtue of being a mage does not automatically make you dangerous. If a mage chooses to misuse their magic or apply dangerous means to their magic, then they are dangerous. Assuming that every mage is dangerous by virtue of being a mage is Chantry propaganda and that line of thinking is exactly what led to the Mage-Templar Conflict in the first place.
So going back to your question: Are mages dangerous and pose a threat to world/society safety? No. Did Bethany do anything that warrants her execution? No. Did she demonstrate any action that would make her dangerous? No. Is there proof that Bethany is a Blood Mage or was taught blood magic? No.
Maybes; Ifs; Mays; and Could's aren't a logical or objective basis for deciding someone's life without any concrete proof and evidence. It would be different if Bethany had demonstrated any problematic signs or symptoms, but she didn't. The worst that she did was fight for her life against crazy templars trying to kill her for a crime that the Circle had nothing to do with.

It isn't subjective assumption unless you want to tell me that every world-threatening disaster we know was caused by mages and because of danger came with being mage is just an assumption not a fact.
First and foremost mage can't chose to not be dangerous with exception of applying tranquility but then person no longer is a mage. Mage is always dangerous because magic is unstable and it not always leads to outcome you want even if you don't mean bad things to happen and being mage means being exposed to demonic influence what is just another threat that mages pose. So what you are saying is nothing more than ridiculous pro-mage propaganda that tries paint mage being dangerous as matter of choice rather than nature of being mage.
"Are mages dangerous and pose a threat to world/society safety?"
Yes , considering that every world-theratening disaster in thedas was caused by mages and because of their powers.
"Did Bethany do anything that warrants her execution?"
Maybe , how do you know she didn't? Are you a omniscient that you know what she did and didn't?
"Did she demonstrate any action that would make her dangerous?"
Yes, she killed quite a lot of people what makes her dangerous (aside of danger tha comes with being mage), but as i said lack of action at the moment doesn't equatate not being dangerous. That Uldred didn't attack you when you first met didn't mean he won't end as abomnation or isn't blood mage.
"Is there proof that Bethany is a Blood Mage or was taught blood magic?"
Was there a proof that Orisno practiced blood magic? No there wasn't , just because you don't have evidence of someone doing something doesn't mean they aren't doing that.She was member of extremly corrupted circle , if she wasn't blood mage or abomnation (what you couldn't have known) she was just tiny part sacrificed to ensue survival of entire machine what is logical thing to do.
As i said fact is poses Bethany is threat to society by mere fact of being mage.
The point here is, if you seriously think Bethany would take up Blood Magic then you need to tell everyone on the forum what you're smoking.
Give ma a logical reason why Bethany is immune on temptations that come with power (blood magic)?