Killing 100 million, or even 10 million, people is extremely difficult in practice, and should leave a lot of evidence. I have a Bachelor's degree in political science, and I have never seen direct evidence of mass killing on such a scale. My personal ignorance does not, of course, mean that such evidence does not exist, but in contrast, the evidence that the Nazis killed 10 or 15 million people is so ubiquitous that I was aware of not only the charge but the evidence and documentation before I started studying political science. I have, for example, actually visited Dachau. So that the evidence for such a massive charge is not readily apparent at least raises suspicion.
I have not studied the topic academically in any depth. However, what little I have studied indicates the evidence for the scope of mass killings is at best indirect, perhaps unsound, and at worst simply fabricated. In The Battle for China's Past, Mobo Gao claims that the "evidence" for the Chinese Communist killing of scores of millions rests on calculating how many people should have been alive in China at some point, subtracting the number actually alive, and counting the difference as mass killing. Not only is this technique unsound on its face, but Gao also rebuts the claim directly: he compares China's population growth India's (where no allegations of mass killings have been made) and finds no significant difference.
Also, despite their problems, both the Soviet Union and China had sufficient economic growth under Stalin and Mao to become world powers; it is difficult on its face to reconcile that economic growth with the incredible economic drain that the mass killings of scores of millions of people would have caused. And these mass killings would have to have been on top of those caused in the Second Imperialist War, unless we to blame the Nazis on Stalin and the Japanese on Mao.
Finally, the capitalist west has been explicitly struggling against communism since its inception. Western scholars, as well as Chinese scholars who, as Gao also claims, desire Western favor, have considerable incentive to interpret any evidence uncharitably. When someone has a good reason to shave the truth or lie outright, we do need to examine the actual evidence most carefully.
As noted earlier, I am not saying here that absence of evidence is evidence of absence; I have not made a thorough search for the evidence. However, this deficiency could easily be corrected by someone who has made a thorough search to send me at least their bibliography.
But my objections do not end just at whether or not mass killings actually occurred. Let us assume, arguendo, that at least some of the charges are true. I would also need to see evidence that the mass killings occurred because of communism per se.
There was a lot going on in the Soviet Union and China. Russia was devastated by the First Imperialist War, which cannot, of course, be attributed to communism. Then, as Trotsky observes in Terrorism and Communism, the Soviet Union faced massive shortages of raw materials because of the Western blockade and the West's prosecution and support of the Russian Civil War; Trotsky complains that the Soviets had to run their entire economy, including their transportation network, almost entirely on firewood. Soon after the end of the Civil War, the Soviet Union faced Hitler and the resurgence in its time of the most powerful industrial economy and the most skilled and disciplined military power, explicitly bent on the annihilation of the slavic people. To prepare to fight Hitler, Stalin required the absolute dedication of the Soviet Union's population and labor. (Very similarly, most Western nations also required near-total popular effort during the war, overcoming substantial resistance and dissent.) Finally, after the end of the Second Imperialist War, both the Soviet Union and China faced a nuclear-armed United States, with at least some US factions explicitly advocating nuclear annihilation, e.g. Patton for Russia and MacArthur for China. While Truman repudiated both, neither the Soviet Union nor China should be expected to rely entirely the United States' charity for their very survival.
There are other things going on too, of course. Both the Soviet Union and Communist China emerged not from bourgeois republics but directly from authoritarian regimes, without even the beginnings of political socialization of democratization provided by a bourgeois republic.
At the very least, those linking provable mass killing to the communists would need to show that these mass killing were not substantially related to the massive effort necessary for weak agrarian economies to industrialize rapidly enough to stave off genuine existential threats.
Finally, mass killing is hardly unique to communism. Mass killing goes back not only to the beginning of recorded history — Thucydides documents any number of massacres — but is shown in the archeological record. Capitalist countries have themselves engaged in mass killing and other comparable behavior, notably the genocide of the indigenous people of the Americas, the enslavement of black people, at least a million deaths in the American Civil War, the First and Second Imperialists Wars and the use of firebombing and nuclear bombing of civilians in the Second, the Irish Potato Famine, the Influenza
It is important to note that the reasons for these deaths are irrelevant in this context. It is simply the number of deaths that opponents of communism claim are relevant, not the reasons or justifications. If no justification could excuse communist mass killings, it would be rank hypocrisy to justify capitalism's mass deaths.
After studying socialism and communism both as an amateur and academic, I see absolutely nothing in socialist and communist theory to justify mass killings. It is possible that Stalin and Mao actually did kill a lot of people, with or without good justification. If so, a good communist such as myself must simply say that we must discover the reasons and causes of those killings, and ensure that they do not recur in the future. Even justified killing is not really acceptable: our goal is the liberation of all humanity, and even one death is a failure.
ETA (5:10 am): Here's another example of capitalism literally killing people: Public Health Takes a Hit Even as Uruguay Prevails in Infamous Philip Morris Investor-State Attack. According to the Centers for Disease control, tobacco kills 6 million people per year. What makes these deaths directly attributable to capitalism is the tobacco companies' powerful resistance to national democratic governments' smoking cessation programs. The perpetuation of these deaths follows directly from the capitalist premise that companies have a property right to profits, and those rights cannot under any circumstances be legitimately abridged by democratically elected governments.