Imo Rielly was wrong. As you said Greig would have been assessed a penalty anyway.
The issue is that this is a group of men playing a game, and I am wondering why we should be thinking it's cool for them to act like school aged boys who have zero self-control.
If NHL players want the benefits that come with playing in a professional league, making far more money than most of the fans, and commanding respect for their talent ... the cost is to act like an adult.
What Greig did was just as childish ... but Rielly used the same poor judgment by attacking him.