Actually, there are VERY few examples of Fallen Jedi, the fact that the vast majority of the organisation stay on the Light Side despite constantly facing danger and combat and difficult moral situations rather goes to show something, yeah?
And Yerssot, no it is NOT about how they look, it is about how they ARE. How can you possibly support such a superficial view of the psychological nature of killing? The effort to destroy a non-living object, even one that vaguely resembles a human, is completely different to the effort needed to kill a human, or other living thing. Practicing one will in no way help you with the other. Actually extinguishing a life is conceptually different. The ONLY way you can practice for it; become more likely to do it; is to DO it. No substitute will work.
So, no matter HOW much the Battle Droids resemble humans, the destruction of them is only equivalent to destroying a machine, and it will not make you more likely to kill living things.