He tried killing his niece in 4, who was pretty young.
I don't have the exact quote with me, but Stef (the writer) pretty much explained that this occurs during the time that Michael thinks Laurie is dead, so he's essentially entertaining himself during her absence. He's not so much interested in hurting the kids (which is why he doesn't kill them when he easily could) but is just messing with Mrs. Mackenzie. His version of Michael (which is a bit different admittedly than the films at times) is pretty sadistic and mean-spirited and enjoys scaring people just as much as he does killing them.