It can be extremely difficult for parents to deal with an angry child, especially one with a violent streak. Most parents get flabbergasted with a child's hostility and are taken aback; at times too hurt and disturbed to even deal with the situation aptly. The important aspect here is to be patient; dealing with such kids needs heaps of attention and perseverance.
Like adults, children could have countless reasons to feel anger. However, the reason should be somewhat proportionate to the degree of emotion.
It's completely normal to be angry, it's even acceptable to be enraged and frustrated, but if your child turns spiteful and malevolent, it might be time to fix the situation.
Be prepared that he or she will take some (or a lot of) time to actually start applying these techniques. However, the advantage is that these techniques will be helpful for life. The child will learn better self-control and his or her temperament won't be a concern again; neither for you and nor for the child himself.
So then what is the solution? Tell your kid that when she comes to you, not only will she get the toy back, but you will punish your son as well. This might catch the child's fancy because simply getting the toy back is not enough incentive in her mind. She seeks to get even and wants some form of justice; so you can assure them that you will make it happen.
Lastly, parenthood is tough, it'll test you at every stage on different aspects. Coping and dealing with every situation is important, but letting it go is equally important. Constantly thinking about something he or she said is simply going to damage your relationship with your child.
✦ Instead of telling your child not to do something, suggest him or her what they can do instead.
✦ Be lavish in your praise and miserly in your criticism.
✦ If the answer to something is 'no', say it firmly, without any negotiations and barters.
✦ Don't scold him when he has just done something aggressive, give him time to think straight.
✦ Sympathize; instead of just saying that you understand, give examples showing that you've been through similar situations and you've felt the same kind of emotions.