Well, no matter what I play I always go with 1 point in Collectivist or Xenophobia, just so I can purge. And I purge everyone that annoys me (even own pops with wrong ethos). I once uplifted a species, then decided "nah my own pops can live just fine one that world" and proceded to purge them all.
Also have you ever gotten the "underground civilisation" event chain?
I decided to do a preemptive strike against them, and the event pop-ups after that told me that I had flooded their caves with nerve gas and sent my soldiers in to kill any remnant - women and children included.
Reading those event files actually made me feel a bit bad for those guys

.
That's when I realised that I am probably a bigger menace to the galaxy, than the endgame crisis.
You can always change the 00_policies file at x/y/Stellaris/common/policies.
Find the line for "purge" and it has these subsequent lines for "purge_allowed_xenos_only".
valid = {
OR = {
has_ethic = "ethic_xenophobe"
has_ethic = "ethic_fanatic_xenophobe"
has_ethic = "ethic_collectivist"
has_ethic = "ethic_fanatic_collectivist"
Expand that to (for example how I have it, you can add whatever you want):
valid = {
OR = {
has_ethic = "ethic_xenophobe"
has_ethic = "ethic_fanatic_xenophobe"
has_ethic = "ethic_collectivist"
has_ethic = "ethic_fanatic_collectivist"
has_ethic = "ethic_fanatic_individualist"
has_ethic = "ethic_individualist"
has_ethic = "ethic_militarist"
has_ethic = "ethic_fanatic_militarist"
has_ethic = "ethic_materialist"
has_ethic = "ethic_fanatic_materialist"
I didn't see a reason why militarists, individualists and materialists wouldn't be subject to prejudices that would lead to purging. I still kept the happiness penalty for individualists though, because it emulates the whole "I know we have to do it, but are we doing the right thing?" philosophical dilemma that such a population would have.