By 1918, the SPD was a fairly moderate party, having split with the USPD, which took, for the moment, the more radical Spartacist with them. Ebert took a decidedly anti-revolutionary stance in response to the October Revolution and when pro-Bolshevik riots and strikes broke out across Germany, the SPD's Interior Minister, Noske, cracked down hard. Nevertheless, during the 1918-19 German Revolution you have former members of the SPD actively fighting for their own revolution against their former comrades.

How German socialism got to this point is itself an interesting question--Schorske's German Social Democracy, 1905-1917: The Development of the Great Schism is probably the definitive book on the subject. The centrist, reformist, and radical factions of the party could work together against the pressures of the Kaisserreich, but once elements of the party became invested in the status quo (and saw real gains through them) while others still found themselves negatively integrated in their own parallel milieu (Lidtke's Alternative Culture is probably one of the most significant books of my undergraduate career--absolutely worth a read), the tensions became too much to bear under the weight of war.