Well, you must understand the historical context... It wasn't like Finland wanted to roll with Hitler. They were forced to, by the aggressiveness of the Soviet Union and the inaction of the Allied forces.
First, the Soviet Union had just become best buddies with the Nazis (Molotov-Ribbentrop Pact and its secret addendum). Second, after demands to cede Finnish territory were rejected, Soviet Union staged a false flag attack and threw eventually overall about a million troops at Finland. The attack was repelled but Finland had to cede roughly 12% of its territory.
Third, the Soviet Union was clearly not done, so what could Finland do? The horrors of 1939 were bound to happen again; Soviets were gearing for war and one could not really expect the weak and submissive Soviet population to make any kind of change for the better within their nation, that much had been taken care of by the death camps, deportations the secret police, tight control of the media, etc. The only actor who could realistically provide substantial help against the inevitable rematch with the Soviet madman was Nazi Germany, a nation run by another madman.
Had USA, UK and the Allies sent troops or at least enough material, or pressured the Soviets to drop their ambitions of annexing a neutral country, Finland would not have needed any help from the Nazis. As it was, Allied help on a meaningful level did not happen. The neighbouring neutral Sweden helped the most, for obvious reasons: if Finland fell, they would be next.
So, yes, it was an existential threat. However, as crude as it may sound, given the circumstances, Hitler being evil or not was surely not even considered.