As posted in this comment I think this question is similar to the several topics tagged suicide -- which are also about the idea of killing, all the same counter-arguments apply here -- and topics like this asking about "antinatalism".
Buddhists have addition reason not to kill others -- it's directly contrary to not harming others.
It's contrary to the first of the five precepts which summarise a code of morality.
There are famous passages from Buddhist scripture like the Dhammapada:
- All tremble at violence; all fear death. Putting oneself in the place of another, one should not kill nor cause another to kill.
- All tremble at violence; life is dear to all. Putting oneself in the place of another, one should not kill nor cause another to kill.
As for the idea that "existence is the root of suffering" see for example the topic, Did the Buddha really say that "life is suffering"?
why aren't there any murderous Buddhists
There probably are, depending on definition.
For example Wikipedia says that the cult behind the Tokyo subway sarin attack was considered kind of Buddhist:
Shinrikyo Aum is a syncretic belief system that draws upon Asahara's idiosyncratic interpretations of elements of early Indian Buddhism and Tibetan Buddhism, as well as Hinduism, taking Shiva as the main image of worship and incorporating millennialist ideas from Christianity, Yoga, and the writings of Nostradamus. Its founder, Chizuo Matsumoto, claimed that he sought to restore "original Buddhism" but employed Christian millenarian rhetoric. In 1992, Matsumoto, who had changed his name to Shoko Asahara, published a foundational book, declaring himself to be "Christ", Japan's only fully enlightened master, as well as identifying himself as the "Lamb of God".
Asahara's purported mission was to take upon himself the sins of the world, and he claimed he could transfer spiritual power to his followers and ultimately take away their sins and bad deeds. While some reject Aum Shinrikyo's claims of Buddhist characteristics and affiliations with Buddhism, other scholars refer to it as an offshoot of Japanese Buddhism, and this was how the movement generally defined and saw itself.
Aleph (Japanese cult)
There have been other historical instances of violence in nominally Buddhist societies -- like, I don't know, samurai, wars, civil wars, anti-immigrant ethnic cleansing.
I see non-violence as so fundamental to Buddhism that I personally apply No true Scotsman to the topic, saying that such thinking or action by (my) definition not Buddhist or contrary to Buddhism.
Buddhism does warn though about, for example, ignorance and delusion -- that (not to excuse it) there's been some ignorance or confusion about what's right, and so on.
Or possibly you might see it as being something like Christian "sin", i.e. contrary to the official doctrine, but found nevertheless in some smaller or even greater extent in some practitioners -- and in people who claim to be, or who "identify as", or who are members of a culture or society which identifies as or is influenced by "Buddhism" or which includes some of its historical iconography etc.