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 : On just about any other topic, I'd be in complete agreement with you.  However, WP:NPOV and WP:Undue weight, taken together, mean that Wikipedia should reflect the attitudes of society, especially (or perhaps, strictly speaking, only) when we can back it up with reliable, independent sources. The Nazis, like other despotic regimes before and since, used language and euphemisms to downplay the evil of what they were doing or, perhaps simply as an act of self-delusion or as a means for the higher-ups to delude/brainwash those who were not the major decision-makers into thinking their actions were not as horrific as they were. 
 :Unless the cited sources can be impeached or someone can show that not only those sources's understandings of the Nazi Holocaust and related crimes against humanity do not line up with the general opinion of history, the recently-added text is perfectly appropriate.  The only reason I can think of not to add it is if it redundant to existing text. 
 :Having said that, a drier, less-opinionated way of saying the same thing could probably be found and such text would serve the same encyclopedic purpose of warning the reader about the different meaning of the term ""euthanasia"". I would have no objection to such a change, and I hope that, as long as the message about the euphemistic use of the term was clear, that  wouldn't object either. // "