The claim is not incoherent or illogical - if a Christian is one who follows Christ and Christ preaches non violence then one who does violence does not follow Christ.
The perceived problem is that this argument is circular - but that is not a problem unless you are trying to define a Christian as a perfect adherent of Christ's teachings.
In Hitler's case you need to ask why he did what he did - did he kill Jews because he believed they killed Christ or because he believed they were biologically inferior and represented a threat to his Master Race because they were a pestilence, like rats?
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