Marx saying "killing people and destroying property solves nothing" still makes me mad. Not just because Marx would never say a thing like that, but also game is a fucking killing spree simulator. Whole game you go into factories to liberate workers, kill templars and destroy their stuff. Why pacify Marx in a game where you kill people with your stabby-stabby mechanism?
If you want to head-canon it, you can imagine that since the animus is created by the sinister templar abstergo company they have sanitised historical figures for propaganda purposes - to that end, if you read the in-game encyclopedia entries on Marx, they're deliberately presented in a condescending dismissive way, as though written by a corporate apologist working for the 'big bad'