Is God able to prevent evil, but not willing?
Then he is malevolent.
Some folks argue that freedom is the highest expression of love. God loves you and made you free. Free to do anything, good or evil.
There are problems with this argument though. We can use Occam's razor to cut god out of the picture. We can ask if the natural limits of physical forms infringe upon our liberty. We note that many religions claim that God does interfere, and that therefore we are not made free.
Then there are arguments to counter these.
This stuff isn't as clear cut as it seems. The definition, possible existence of and origin of evil are interesting theological questions.
I'd rather spend most of my time acting for the good though instead of thinking about all the abstract stuff. Well, except for posts to slashdot.
-Peace and love-
The Tao is like a glob pattern: used but never used up. It is like the extern void: filled with infinite possibilities.