First thing Hitler did was arrest the socialists and communists, then make unions illegal.

Not that I agree with the comment above but that means nothing at all on itself.

One of the first things the Socialist government did was violently put down a communist coup. The communists would have abolished democracy ASAP and purged the socialists if they ever took power.

Fact is that extremist movements will crack down on anyone that tries competing with them for power. Ideological affinity hardly matters.