I don't see it as lying or even untruthful; to me, it is an age-appropriate way of beginning to explain something that is conceptually complicated (and I accept that not everybody understands in the same way).
There was a thread last week about the Bulger case. A poster linked to a piece from the Independent about children who commit very serious offences and how they are rehabilitated which set me thinking.
Though one tends, understandably, to react with shock and anger faced with such offences - and it feels easy to label offenders as bad, wicked or evil - moral judgement isn't particularly helpful in coming to an understanding of how people come to commit awful offences (or the possibility of rehabilitation).
I do regard extreme incapacity for empathy as a mental deficit resulting from inborn unfortunate brain structure, or adverse life experiences which effectively 'damage' the brain (or a combination of both), so that it is unable to function in a way that results in behaviour within social norms.
This dysfunction seems to me to fall into a broad category of illness of thoughts and feelings, as opposed to illness of the body, and I feel that would be a legitimate place to start explaining extreme 'badness' to a small child, without having to bring 'badness' into it at all. It can be expanded upon as the child is capable of processing more, but in the meantime covers it without being frightening.
That said, I'd be totally open to being told this is all way off the mark by an expert. DD heard part of an item on the news about a mother who killed her children and it made me think that at some point it will be necessary to begin to explain some of the horrible things that happen in the world.