To quote Fury,
Ideals are peaceful, history is violent.
Everybody has their ways to cope, which I understand, but this "let's change ourselves to eliminate terrorism" is rubbish.
Terrorism will always exist, and I am sorry to say this, but that part of the world doesn't seem super eager to change and become less violent.
Terrorism and Terrorists need to be stamped out. If the US and Europe and whoever else tomorrow suddenly left them alone, they will still continue to try and cause harm to us. They will smell our weakness out and do whatever they want. They do not understand peace. They understand violence and brutality.
Flowers and Candles don't mean leaving ISIS alone.
As mentioned they are symbols of remembrance and vigilance.
As example, consider two slogans that resulted from the Holocaust - "Never Forget" and "Never Again" - both of which are remembrances and calls for vigilance. The result of these (potentially misguided policies) is that Israel's right wing believes vehemently in "First Strike" tactics toward any potential threat.
This is, arguably, an example of over-vigilance - but the point stands regardless - that eschewing the apathy and ignorance of forgetfulness - not allowing the past to become less than a memory - can be a call for vengeance and retribution.
In this way, a flower of remembrance, of vigilance, is also - like the Red Rose - symbolic of passion, and even violence.