"guys be nice to nazis or there will be more nazis"
yeah no
It's not a matter of treating them "nicely," but treating them "fairly" (to at least a certain degree, and also based on the present actions of the individuals in question). Because if you don't, then you run the risk of legitimizing their ideas through martyrdom. Furthermore, if you don't focus on the ideas themselves, you risk muddying the definitions, which will make it easier for the ideology to "recruit" new followers over time.
Ostracization and derision is treating those people fairly.
If you always think that what you believe to be obvious will continue to be considered obvious by the world you live in by default, you're simply naive.
No bro, believe it or not but ethnic cleansing is a pretty shitty principle to base an ideology off of. Shocking, I know. ---