I hope I'm not not regurgitating false information (I'm finding it difficult to find a reputable primary source, since there's a lot of clearly pro-ketamine sites), but I was under the impression that one of the reasons that ketamine has been used both in people and in applications like as a horse tranquilizer is that it's safe dosage range is considerably wide, presenting some margin of error over similar drugs.

Obviously it's more than he should have been given (and tbh am pretty uncomfortable with it being given here at all), but my decidedly non-expert opinion (and the official autopsy statement that the "ketamine in McClain's blood was at 'tolerable levels' ") is that there's all sorts of red flags about this situation.