The Finnish social-democrats never wanted to join the Soviet Union, they weren't Bolsheviks after all. In fact, they were the most staunch independentists, they resisted both the Czar and later the Kaiser, while the conservatives true to their habits gladly licked the boots of both.
In fact, the conservatives were brought into power by Russian arms in the summer of 1917. Only after the October revolution did they change their allegiance - not to Finland! - to Germany. To secure their German satellite kingdom the sons of the upper-class then proceeded to massacred tens of thousands of poor tenants and workers, the wretched and the hungry, who had only fought for their right to bread and independence.
This view is known in Finland as the "Red Truth"; there is also a "White Truth" and both are in many ways incomplete in explaining the goings-on in Finland circa 1916-1919.
The conservative royalists were ready make Finland a German client, but by the deals the Reds were making with the Soviets, Socialist Finland would have been nearly as dependant on Petrograd/Leningrad as Royalist Finland from Germany. Maybe even more so, considering Finland's geopolitical position.
Internalizing the idea of independence was hard to both sides, necessarily: it is not like the Finns had tried being independent before... Coming from the Finnish historical background, securing the support of great foreign powers in exchange for political concessions was a logical thing to do, even if in hindsight both sides can be accused of selling out "Finland" or the Finnish people.
Both sides committed atrocities, par for the course in civil wars, and the Reds actually started outright massacres. The White Terror was more widespread and organised, but one has to also remember that a lot of the deaths at the prison camps in 1918 are as much attributable to diseases and a generally poor food situation as to actual war crimes. There is, for example, a recent study that estimates around 40% of the fatalities at the camps being caused by the Spanish Flu.
Peter Hillock said:
Whether the Whites could have won the Finnish civil war without the German division that landed is a question people have argued about since, oh, 1918. They've also been arguing about whether Finland was a German puppet or an ally (WE have allies; our enemies have puppets) between April and November 1918.
Assuming no German troops in Finland, but OTL support for the Reds by the Bolsheviks, the Whites would have still won. The Battle of Tampere broke the back of the revolution, and after that the Reds were fighting a losing battle. Quite simply, they never could form a real army while the Whites soon proved to be superior in terms of leadership and organisation.
Without the Germans, however, the war could have dragged on for months longer, the Whites slowly pushing the Reds south and east. There is no doubt that a longer war would have created more atrocities on both sides, during the war and after it. However strange it may sound, the German intervention, by facilitating the Red downfall, saved the Finns from themselves and prevented the loss of thousands of lives more. Finland never fully recovered from the Civil War. In an ATL without a German intervention recovery would have still been much, much harder.
Finland was a minor German ally on the very brink of becoming a true German puppet. The process was ongoing, and not yet finalised by the time Germany lost the war. Finland was peripheral to German interests: this gave the Finnish conservative leadership some leeway. It is hard to say how much this leeway would have continued to exist in a CP-victory scenario.
Peter Hillock said:
Ultimately the proof of the pudding is in the eating. By the 1930s the Finland that established itself against the will of its own people was the most democratic, stable, and least screwed-up of the new states. Well, tied with Czechoslovakia, which got set back by the triumph of its "people" in 1948.
It is nothing short of miraculous how well the young nation recuperated in the 20s and 30s. We owe a great debt to the moderates of the time, to the people who moved swiftly in demanding pardons for the Socialists, closing the prison camps and pushing improvements like the land reform. With a longer, more bloody civil war polarizing the people even more, Finland might have never grown up to become a stable Nordic democracy.