Karl Marx's Philosophy of Communism never condoned killing.
In fact Marx was against it.
Aside from the aspect
@TerranceL mentioned regarding Marx's stance on violence, even if we want to go with the fact that "he later changed his stance", the reality is, communism has always gone hand in hand with violence because in order to make it work, you need a very heavy handed government.
Marx's theories were such that maybe they sounded good on paper (to some people, certainly not me), however, they require the vast majority of a population to be like-minded and stay like-minded after they realize exactly what the system entails.
Communism falls flat on its face in a matter of months without heavy handed government.
Here's the reason why...while people may have been on board with the ideas, and even voted communists into power initially, it doesn't take long for people to sour on the idea once they realize:
"
hey, wait a minute, I'm busting my hump everyday as an engineer, that guy tearing tickets at the movie theater has the exact same lifestyle I do, as does the guy who I know is faking illness who I see with his friends at the coffee shop every day. Well, if my lifestyle is going to be the same as their no matter what I do...then I don't need this stress, I'll just tear tickets at the theater or fake an ailment and go hang out with those guys"
If too many people do that, obviously the whole system breaks down because you still need engineers and all of the highly-skilled professions to exist for society to work (you need someone to fix power lines when they're down, to work in hospitals, etc...), A society can't function with everyone working the easy jobs and nobody working tough or high-skill jobs.
So, the government has a few options, it can either start incentivizing the more demanding jobs (meaning they're straying from the roots of what communism's about), or they can send in people with rifles to say "hey, we know you were an engineer before and we know you're capable of working, so we're not going to let you work at the movie theater or sit around at the coffee shop all day...back to work at the power plant!"
Where capitalism (even with some of it's flaws) creates a race to the top due to the more demanding and highly-skilled positions equating to a better lifestyle, communism creates a race to the bottom because when every position creates the same quality of life, the easy low-skill jobs are the most desirable.
A good working example of the differences & outcomes of each system on an apples-to-apples basis would be to look at some side by side pics of East & West Germany when they were still divided.
Same people, both recovering after a post-war era, similar demographics, etc...
Within a few years, East Germany looked like every other eastern bloc dictatorship (crumbling buildings, poorly maintained roads, burnt out cars on the side of the road, armed soldiers on street corners, etc...), West Germany's economy was rebounding, industry was coming back, the cities looked active and vibrant... ...and like with so many other examples, you had people willing to risk their own lives to get from the communist side to the capitalist side.
