I think people are being unduly unfair on the social attitudes of the late 19th Century Western public. A very large number of people WERE offended by such brutality and thought very badly of it, but for a large part, people just didn't know about it. Or if they did know, it wasn't given enough media attention for them to feel enraged enough to demand action. But when a concerted campaign did happen - see the slave trade ban for example - public concern did go through the roof. It just was that there wasn't much impartial reporting from Africa in this day and age.
In the case of the Congo, Leopold had a huge and very successful propaganda campaign to suggest firstly that he was being humanitarian in the Congo, then that nothing bad was going on, and then that bad stuff was going on but he was striving to fix it. It was only due to the meticulousness of Edmund Morel as a journalist that this was successfully combated (eventually). The terrible case of the Herero simply didn't have a Morel to draw attention to it.
As for the overall discussion, complete depopulation did certainly take place in the Western part of the territory (excluding the enslaved of course). It's quite possible that some ethnic groups were made extinct - we don't have sufficient knowledge of the different pre-colonial ones to know. However, the massive size of the overall territory is just too large, and the Belgians did not have enough reach, for them all to be killed.