The social democrats gave military hardware to the freikorp to put down the communists and arranged the assassination of Rosa Luxembourg. The communists said the SPD were creating the conditions for fascism, and they were correct. Hitler came to power when the SPD ran a conservative candidate as their "best shot at stopping the nazis". They won. That conservative proceeded to staff the government with nazis while cracking down on the left until it was trivial for the nazis to engineer a coup.

Regarding the molotov-ribontrope, if thats an alliance of convenience, what is every capitalist power arming Germany and splitting Czechslovakia between Germany and Poland, while ignoring the USSR's offer to invade Germany with 1 million men?

(yeah I'm posting wikipedia, I don't feel like spending the rest of my sunday night hunting down the specific books on libgen that are cited on each page, suffice it to say these are events which did occur, and even English Wikipedia doesn't deny them, the problem with a lot of these pacts is that they are named different things by different countries)

Four-Power Pact (worth noting france did not ratify)

Hitler Pilsudski Pact

Anglo German Naval Agreement

Anti Comintern Pact since this was between Japan/Germany there's less amnesia about it

German/British and German/French 1938 non aggression pacts are usually just called the Munich Agreement in English language sources to obfuscate what was happening

German Romanian Economical Treaty March 1939

German Ultimatum to Lithuania in March 1939 seems a bit unfair to characterize as a treaty tbh

"Pact of Steel" with Italy 1939 Now THIS is actually a military alliance pact, which is what people keep characterizing molotov-ribbentrop as

Danish-German non aggression pact May 1939 this one isn't wikipedia

Germany Estonia N-A-Pact June 7th 1939

German-Latvian N-A-Pact June 7th 1939

Molotov Ribbentrop Pact

Also it's worth noting that even though the USA never signed a non aggression pact with nazi germany, american capitalists were quite fond of funding nazis, doing trade with nazis, and even hosting nazi party rallies in the United States (such as one in madison square garden in 1939). And there was even a fascist attempt at a coup against FDR, that was thwarted by smedley butler being a whistleblower.

What makes people freak out about molotov ribbentrop is the agreement to partition states. the reason for this is because the USSR needed time to deconstruct factories and move them east before the luftwaffe could bomb them. The USSR made a calculated risk that by dividing poland they could move the future war front far west of where it would otherwise be if they refused a non aggression pact and just got invaded right away, and at the same time, move their war industry far east of the war front. Of course by making this calculated risk they gave an easy propaganda victory to all bourgeois nations who experience collective amnesia about every other non aggression pact, as well as their categorical refusal to enter into collective security against fascism with the "bolshevik menace" in the 1930s. It is somewhat similar to how USSR tried to join NATO in the 50s and was refused. The bourgeois nations never wanted collective security or peace with the USSR, even when it was on the table. In particular the USSR only invaded finland because it was the most convenient route to invade the USSR, and the Finnish (a future axis power) wouldn't sell them the land that the nazis eventually used to invade.