Speaking as someone with some experience with marital arts, you have to be careful when "using anger" when fighting. As martial artists, not thugs, when competing against each other we always strive to fight with control, and always with the safety of our opponent and ourselves in mind. That being said, you can often see people get frustrated, either because they're losing badly, or just through fatigue, and this impacts your fighting. Yes, you often strike much harder when your upset, but your lose focus and concentration, your technique suffers badly.

This means that you are more open to your opponent's attacks, and you may also attack and move in ways that are more likely to injure yourself (e.g. stepping badly and breaking your ankle). It can also lead you to being manipulated and provoked by your opponent more easily, and do things that you would otherwise see through if you were in a more stable state of mind.

That being said, there is some use in anger when fighting, provided you remain in control. (disclaimer: I have never been in a life-threatening situation myself, but there are numerous historical accounts of people in swordfights and other combat situations that detail the mindset of the people involved). If you remain calm, controlled, and focused, you can leverage your aggression in a precise, focused manner to dominate your opponent. (Some of the best tournament fighters have a mentality very similar to this, and yet are able to control themselves perfectly to fight safely. These people are utter beasts in the ring)

This is very different from entering a fight in a blind rage; even if you do win, you're just as likely to die from all the little mistakes you wouldn't have made with clear head. And the line between them is very thin, making "get angry" as a basic tactic very dangerous, and not very useful.

Which brings us to Ruby. Yes, I'm sure that mature, professional Huntsmen would be well able to fight with a clear, focused anger in their strikes, but Ruby? A 16 year old girl with a string of failures and deaths haunting her steps? I'd say she'd be more on the "blind rage" side of things. This is a very dangerous way to fight, and it will be interesting to see if there are any repercussions for this