How do I handle my six year old's aggressive behavior towards his one year old brother?

My eldest son has been up to this point a very mild tempered, well behaved boy. It was almost too good to be true. But recently, he's been pushing the boundaries and now become aggressive towards his brother. Two days ago he slapped his little brother and just now he pulled a mat from under his feet, resulting in a big knot on little brother's head. I don't want to approach him negatively our have him resent his little brother.how should I approach this, when respect for oneself and others has always been instilled in him?