You don't think it's evil, or it's definitively not evil? What if the driller was eventually consumed by the drilling fantasy, bought a cordless drill, and went over to his neighbor's house with the intention of drilling out his eyes. So, it's still in his head, still a thought. While walking over with the drill, he's still not evil? (Buying a drill isn't evil, nor is walking to a neighbors house, I think we can agree. Since we can separate thoughts out, it doesn't matter why he bought the drill or walked to his neighbor's house.)
He arrives, knocks on his neighbor's door, and there is no answer. Turns out, the neighbor has moved on short notice. So, the driller goes back home, without harming or murdering any other person. If actions are all that matters, say if you were looking on at the scene, you would probably imagine he was going over to help with home improvement or was lending him the drill. No evil. If you're inside the drillers head, you see the gruesome fully realized plan. Still not evil?
I think there's 2 vectors on the drill example.
A person who is angry has an idea involving a drill. He then meditates on this and realizes that doing so would be wrong on a number of levels. He abandons the idea, having figured out what was wrong with this idea.
OR
A person who is angry has an idea involving a drill. He dwells on this, getting angrier and angrier. he buys a drill. He makes plans on following through.
As Celebrim mentioned the 8 path thingy, the first 2 steps were right understanding and right thought. You can't have any of that if you don't contemplate WHY an idea is wrong. That means you have to actually think about stuff and destroy a negative idea. This is where a negative idea does NOT make the thinker evil.
In the second example, obviously the person is reinforcing this negative idea. This is where a negative idea CAN make the thinker evil.
Because it's all in the thinker's head, there's not a lot of proof of a crime (equating crime to evil). That in turn makes it hard to convict (being judged by others). This is why the base metric is in their actions. Because whether thought is/becomes evil action or not, evil action is where the rubber meets the road.
It is the philosophical compatibility point. We may not agree where "evil" begins, but we can probably agree on some "what is evil" points.
If we're just talking basic criminal acts like stealing, killing, it's probably cut and dry. If we're there's self-defense or starvation as a factor, it might get fuzzier. That's motivation. Which is something the legal system struggles with as well. Motive isn't supposed to matter in court, but it does to humans.
Celebrim's example of the good is defensive, evil is offensive was a pretty good explanation of the trend towards playing evil. A good defense is a good offense, as some football guy used to say. Evil tends to get the good offense.