Before Luxemburg broke with and was later murdered by them, the SPD had the orthodox centrist current led by Kautsky, the left current of Luxemburg and the revisionist current of Bernstein. I understand that anarchists considered the SPD and the Second International to be bourgeois from the beginning. In reading about the history of the SPD, one is stuck by the foreshadowing of Leninism/Stalinism in the actions of the SPD in the repression of communists and the working class.
Crimes of the SPD:
1. Supported World War I
2. Took part in the repression of German communists and the murder of Luxemburg and Liebknecht
3. Supported von Hindenburg who democratically handed over power to Hitler
Any others?
The blame for the majority of the SPD voting for the civilpeace can go to the nationalism of the trade unions.
Ebert was personally shocked on hearing of the murder of Luxemburg, which of course means nothing, but perhaps the states' machine repression wasn't fully in the SPD's hands. I mean, even Kautsky was at one point arrested.
I don't know about the SPD's support for Hindenburg and if that paved the path to nazism.
Anarchist were participants at the SI congresses until 1893 when they got expelled, so I doubt "the anarchists" considered the SI bourgeois from the start, and even after that probably nationalist, reformist, etc. at worst, but so did the marxist current within the SPD.
Luxemburg, Liebknecht, etc. didn't actually leave the SPD until the USDP formed in 1917 (a split which according to wikipedia the Spartakusbund rather didn't want: Daraufhin gründeten die Kriegsgegner in der SPD im April eine eigene Partei: die USPD. Dieser schloss sich nun auch der Spartakusbund an, obwohl er die Parteispaltung nicht gewollt hatte.)