Hearing his words, Peter clenched his fist. At this moment, his stone heart directly acted and stopped all of his rages. Unlike his Mind of a Hero, his Stone Heart focuses more on strength. If Peter is capable of defeating his opponent, he doesn't control Peter's emotions instead adds some into it.
Stone Heart was the reason why Peter was dead set on killing Morgan otherwise he could've simply ignored it after the warning from Council. But, his Stone Heart forced him to take action by increasing the killing intent in his mind.
He found he can't control his Mind of a Hero or Stone Heart but he can bring a balance in these two. Of course, only when one of another tries to fight back. In the previous case, his Mind of a Hero didn't show up. And, so his mind was completely overwhelmed by the killing intent.