Source

The French Bankers Too greedy?
The French trader, Jerome Kerviel, at Societe Generale that in the end cost Societe Generale 4.9bn-euros ($7bn; £3.7bn) was at the beginning of the year in profit of 1.4bn euros.

The bank says:
”
Societe Generale said Mr Kerviel’s experience in administrating trades enabled him to bypass strict risk controls.

It said he invented deals that, on paper, balanced out his bets.
”

Kerviel says:
”
Mr Kerviel said he “did not believe” the bank’s senior management would have been unaware of the risky bets he was taking, according to testimony published in the French daily Le Monde.

“It’s impossible to generate such large profits with small positions, which leads me to say that when I’m in the black, my superiors closed their eyes about the methods and volumes committed,” he was reported as saying.
”

My personal thoughts:
I think it is common practice for the big banks to do some investing with their own money to some extent. In good years it is a comfortable extra earnings.

He did not do it for personal gain. He would probably have gotten a fat bonus if he made a lot of money. But would it not be noticed that he had done something he was not allowed to do when he realized a big profit?

On the other hand he does not say that it was something he was told to do as part of his job and that he was instructed by management to do it.