Similar to Hackerdashi's answer, you are probably
Known since the ancient times
Knives/blades are very old tools.
I'm sort of a chameleon
Blades take on different shapes depending on their intended use.
Sometimes I am ugly
Open for interpretation, but ugly could be either the shape of the blade being unpleasant, or the use of a blade to do bad things.
And sometimes worth a million
Some blades are very valuable and expensive, such as the kukri.
This value, however, is nothing compared to what I can do if not handled with care
Knives are dangerous. Need I say more?
You use me to leave a mark on a tree
Well, that's one way to use a blade...
Or to free life of impurity
...and that's another way.
My family is huge, I can't say I'm lonely
As mentioned earlier, there are lots of different kinds of blades.
But only the lucky see the pretty side of me.
Better to be on the right end of the blade. Although, perhaps Eutherpy is being a bit dark for suggesting that using a blade for such things as "freeing life of impurity" is pretty. :P