I don't think categorizing only children from those with siblings in this way is very fair. Not all lone children are as you describe, nor are those with siblings.
Children often behave according to the way they have been brought up and it's in their childhood they learn about sharing, compassion, consideration etc. You'd be better looking to what the parents of the children are like first and then making up your mind.
Lone children have a bigger chance of their full parent's attention, of more one to one time. Perhaps this brings more security and a secure child will be a happier child. Often it's unhappy children that show the negative traits you mention.
Having to struggle for mom's attention, or not having enough money to buy that pair of football boots may just be making little Jimmy resentful!
I know several large families with 4+ children,and they are the most loving considerate of families you could meet. The sense of "family" is great and their children are far from selfish or manipulative.
I don't know where "peaceful" comes into it all really. It's never peaceful in a home with several children and if there are many of you, perhaps you have to learn to shout the loudest! That said, I was brought with two siblings and my sister was a very peaceful, calm and tolerant child, whilst sadly, I was the opposite!