Goal #1 of a radical organisation: create a division. Your narrative is that you represent a downtrodden demographic - "us" - whose misfortune is caused by those in power - "them".
Goal #2: invite hatred and repression The worst thing that can happen for you is for your division to dissolve. You have to work to make membership in your "us" group a stain on its perceived members.
Example: Islamic terrorism. Only a small group of radicals is actually involved in operations, but because they are so deliberately monstrous, and constantly claim to represent all Muslims (factionalism notwithstanding), the stain of their deeds gets spread on all Muslims. This drives a wedge between "Muslims" and "non-Muslims", eventually making that the #1 defining identity for any given Muslim.
The worse the atrocities e.g. ISIS commits in, say, the EU, the louder the voices that demand society be protected against islamic radicalism get. People start to demand that Muslims take responsibility for stopping violence committed by "their" brethren. Muslims start to lose standing in society. This breeds resentment, and reinforces the ISIS message that Muslims are a repressed group who need to fight back. This increases radicalisation, which increases violence, which increases repression, which increases resentment.
By deliberately being monstrous, a radical group can leverage a society's self-defense mechanisms as a recruitment aid. So I'm sceptical of hard-line solutions to these kinds of problems. They seem more likely to make things worse than better.
What should be done, then? I sure don't know, but some things I think should be considered:
1: Do not acknowledge the division. Certainly do not reinforce it. It is an illusion that serves the radicals.
2:Fix the root causes. The radicals' goals are irrelevant. There's no Muslim majority fervently longing for a Caliphate. There's no widespread desire for a whites-only USA among the white conservative-leaning population. What people care about is food on the table, opportunities to progress in life and better their lot, and not being singled out for ridicule and punishment day in, day out. It's the economy. When the economic prospects for large demographics tank, bad things start to happen.
3. Hope for the best. It may take a generation to see meaningful improvement in quality of life for the disaffected population under risk of radicalisation. In the mean time, the political apparatus needs to stay afloat long enough to see those results.