People do evil things because they don’t seem evil to them. This is about as far as we can go to understanding the mindset because it’s largely irrational. The people on the German plane didn’t deserve to die. As far as we know, none did horrible things to others and yet they were killed.
By definition the word the word means “morally wrong or bad,” but that leaves us debating the question of morals. All you have to do to realize that morals vary widely is get into a discussion of politics or religion at your next social engagement. People have different beliefs. So, we’re left with trying to define goodness–the opposite of evil–with a zillion perspectives. We all individually come to our view of the world from our own experiences, which includes those things we were taught and those things we’ve experienced personally. Beliefs about evil and goodness are based on the lives individuals live. If religion is included in your world view, you’ll include those beliefs in with your definition of good and bad.
All we can do then is consider the opinions of the masses. Most people think killing is wrong, but there are exceptions. Killing isn’t wrong in war, when the enemy is trying to kill you, too. Killing isn’t wrong to protect yourself or someone else. It gets sticky, however, when we consider the definition of “threat” and the emotion of fear.
Fear is pretty much universal. Most people are afraid they aren’t going to have what they need and even “need” is defined differently by various people. Humans fear loss–of stuff and important people, of position. Lots of fear everywhere. Fear leads to over-reactions and sad out-comes. Fear causes us to shoot people we feel threatened by, even though the light of day later indicates they weren’t threatening us.
Most people from most circumstances would agree that those things hurt or damage others are evil. Downing a plane filled with individuals not in a warzone–that would fit the description.