It's a giant leap because one thing is to see an angry Anakin killin people, and another thing is to be rather confused, but still a good person, and the second later you are bad bad bad because someone told you to be so, and you kill people because you're angry with noone, and someone told you to do it.
Killing children is not a giant leap. The giant leap is turning a doubting guy to a deliverate assasin in just five minutes of movie because he's been brain washed