There are some people who pride themselves on not taking a lot of shit from other people, and Will's definitely in that category. It's not about him being scary or unpredictable, it's knowing that if you go after him, he's going to fight. That's his reaction. Some people flee, some people surrender, but Will (and Brad) don't do that. We can contrast this with JJ, who when embattled, will ultimately get bitchy, or pout. That's a lot different than how Will handles things.
As for the dour look, it wasn't an "I hate you" malicious look, it was a "you need to stay out of my business" look. A warning look, if you will. Very common in my family, and it usually registers and the topic is changed.