Can a person choose evil? Of course. But can a person choose evil? Not evil as such, no.
That answer doesn't sound right the first time you hear it. If you're used to these sorts of questions, the words "as such" might even signal some sort of ivory tower rigmarole too enamored of a philosophical theory to notice the plain facts of everyday life.
The "as such" may be rigmarole, but it's necessary, and it takes much better notice of the plain facts of everyday life than most other answers.
If you choose something -- if, that is, given a choice between two or more options, you pick one -- then there has to be something about the choice you choose that makes you choose it. This "something about the choice" that makes it desirable -- in fact, more desirable than every other choice -- is called "goodness." "The essence of goodness consists in this, that it is in some way desirable."
This is easy enough to accept for certain kinds of choices. A good choice is a good choice because it's good. Even a bad choice can obviously be a "lesser good;" a good night's sleep is certainly a good thing, even if it's just as certainly not as good as nursing someone through the night whom you have a duty to care for. A good night's sleep is desireable for its own sake.
But other choices are less obviously good under any aspect. Isn't it twisting the meaning of goodness to say that a murderer chooses murder because murder is in some way good?
My answer is that it is twisting the meaning of chooses to say that a murderer chooses murder despite murder being in no way good. If, any way he looks at it, the murderer can't see anything good -- which is to say, anything desirable -- about murder, then how can he choose it, since choice is an expression of desire?
There's an argument against my position* that says that, as a matter of empirical fact, people make evil choices with no reference to the good. But that isn't contrary to my position. Malevolence does exist. What I'm saying is that even someone who chooses something because he correctly perceives it as evil is acting because he judges that there is something good, something desirable, about choosing evil. Which means that, even if someone thinks he is choosing evil as such, or evil for the sake of evil, he is choosing evil under the aspect of good, evil for the sake of some good.
You can deny that the essence of goodness is that it is in some way desirable, but I don't think you can deny it and still wind up with a coherent understanding of human will.
* I say "my position," although I've lifted it from St. Thomas and others, because St. Thomas and others aren't responsible for my errors and confusions.