No it isn't.
The BBC's headline is committing the common (and sloppy) mistake of confusing a correlation with a causational relationship. Here is what the article actually says:
The length of a man's fingers can reveal how physically aggressive he is, Canadian scientists have said. The shorter the index finger is compared to the ring finger, the more boisterous he will be…
The trend is thought to be linked to testosterone exposure in the womb.
In other words, finger length and aggression are both thought to be affected by a common factor (you'll note I didn't say cause), namely testosterone. But to say that finger length is the 'key to aggression' (albeit in quotes) is like saying that the coldness of my toes is the 'key to the snow' in my garden.
Time to get off my hobby-horse and put on some thicker socks.