The title sounds clickbaity, so tl;dr for those looking for a summary: it's probably clickbaity because they don't want to be seen as making a direct accusation they could be sued for. The monkeys suffered in various ways during the trials (all of which are detailed in the article, including but not limited to bloody discharge, loss of motor control, brain infections, edema, etc) and had to be euthanized. Musk claims they were all terminally ill anyway and none of the deaths were from the implant, but this seems to be a blatant lie.
And now he wants human volunteers. Isn't that fun.