Peter Kropotkin's stance on WW1 was that only an international worker's conference could bring about peace, but he felt this wouldn't be possible until German military power had been sufficiently smashed, otherwise German imperialism (which was, indeed, the most aggressive party in the war) would set the terms of the armistice. So he argued that workers in France and Russia shouldn't sabotage the war effort while those in Germany should, until peace could be made possible.

It was a pretty wrongful view anyway (and partially motivated by his personal Francophile and anti-German sentiments), no wonder every other major anarchist at the time criticized him for it, but saying "Kropotkin supported the war that killed millions" is a terribly misleading way to put it; and i don't even need to say that using this as an excuse to dismiss all of his works is just juvenile nonsense.

and worked for the liberation of mankind instead of supporting a war that killed millions.

Marx personally supported Bismarck (a ruthless, monstrous man responsible for the deaths of hundreds of thousands) in the Franco-Prussian War for self-serving reasons (he argued a unified Germany would give more leverage to his ideas over Proudhon's in the socialist movement and also argued that German's worker were just better than the silly French, you know?). In his 1848 article "Democratic Pan-Slavism" Engels supported, with Marx's approval, the Westward expansion of the US because in their view the "lazy Mexicans" and the Indigenous weren't being productive enough with the land (see pg. 261 of this pdf archiving the Neue Rheinische Zeitung, funny enough it was even on an anti-Bakunin piece).

I doubt you'll dismiss all of Marxism over this, and so you shouldn't do the same to Kropotkin.