This morning, the New York Times published an investigative report on the deaths of 21 people at a United Nations school in Gaza on Wednesday. The school, located in the Jabaliya refugee camp, was serving as a shelter for more than 3,000 Palestinians who had fled their homes. The evidence so far indicates that Israeli artillery fire killed the victims.

If you see Israel as a bully, the Jabaliya incident looks like a massacre, one of many deliberate strikes on civilian targets. If you see Israel as inherently moral—the world’s only Jewish state, democratic, besieged, and acting in self-defense—the natural inference is that somebody else, no doubt Hamas, must be responsible for the error. But the evidence suggests that the truth may lie in between: Any civilized country could commit such an atrocity. That doesn’t make it any less an atrocity. In fact, it makes the atrocity even more disturbing.

Here’s the evidence, as compiled by U.N. investigators and the Times. The U.N. operates many shelters in Gaza and regularly sends their GPS coordinates to the Israel Defense Forces. The IDF had the coordinates of this facility. Like other such facilities, it had a U.N. flag. Based on the munitions, shrapnel, calculated trajectories, and testimony from two dozen witnesses, U.N. investigators concluded that three shells hit houses across from the school, two hit a classroom where refugees were sleeping, and one hit a courtyard where men were praying. According to Times reporters Ben Hubbard and Jodi Rudoren:

… the number, trajectory and blast marks of the shells all point to artillery. United Nations officials said shrapnel from the site had codes matching unexploded shells recovered from other schools that munitions experts identified as 155-millimeter artillery shells. Damage indicated the shells came from the northeast—where Israeli artillery units are stationed on the hills outside Gaza’s border.

Israel received extensive photographic evidence of the destruction from the U.N. on Wednesday. At that point, an IDF spokeswoman said militants had “opened fire at Israeli soldiers from the vicinity” of the school and that Israeli troops had “responded by firing toward the origins of the fire.” The Times says it emailed a map of the shelled locations to the IDF’s spokesman, Lt. Col. Peter Lerner, and “asked him to point out where Israeli forces were operating, and from where in the 200-yard radius around the school they saw enemy fire; he did not respond.” When the Times contacted the Israeli general whose committee investigates such incidents, he “said that he did not know the details of what happened in Jabaliya because the troops involved were still fighting and therefore had not been interviewed.” Both the general and the IDF spokesman “refused to say what ordnance was deployed.” The Times also asked to speak with representatives of Israel’s Coordination and Liaison Administration, which manages communications between the IDF and international organizations. These requests “were not granted, and detailed written questions about the Jabaliya episode were not answered.”

If the IDF caused these deaths, how does that culpability square with its stated commitment to spare civilians? How could this have happened? Here are four answers.

1. Good intentions. After the Gaza war of 2009, a U.N. report accused Israel of deliberately killing civilians in a strike at another school. Israel insisted that the civilian deaths were accidental and that its forces had aimed at militants firing a mortar from a site 80 meters away. The official in charge of the report, Richard Goldstone, later renounced the accusation that the strike was deliberate. Israel treated this as exoneration. But deliberateness isn’t everything. As the Times points out, Goldstone never retracted the report’s conclusion that the 2009 strike “cannot meet the test of what a reasonable commander would have determined to be an acceptable loss of civilian life for the military advantage sought.”

The same mentality is at work today. By the Times’ count, six U.N. shelters have been hit during the current war. IDF officials insist that none of them was targeted. But at some point, you’ve accumulated so many accidents and deaths that your good intentions are no longer a sufficient excuse. In fact, your fixation on good intentions is blinding you to your recklessness.

2. Weapons drift. When you focus on intentions, it’s easy to lose sight of tactical decisions that endanger civilians as a side effect. High on this list is the IDF’s shift from guided missiles to artillery. Based on the U.N. review and its own reporting, the Times says the fatal hits in Jabaliya “were likely to have come from heavy artillery not designed for precision use.” Such artillery is “considered effective if it hits within 50 yards of its target.” That margin of error obviously increases the risk to civilians.

A human rights lawyer tells the Times that no matter how hard you try, “You just can’t aim that weapon precisely enough in that environment because it’s so destructive.” From the standpoint of good intentions, that’s an excuse. But morality isn’t just about where you aim. It’s also about the weapon you use. It’s easy to tell yourself that you aimed as well as you could, when the fatal decision was to use a weapon you couldn’t have aimed any better.

3. Context drift. The anonymous Israeli general interviewed by the Times says that just before the school was hit, “Hamas people were shooting at” Israeli troops who were trying to demolish a Hamas tunnel. The Times asked the general whether it was OK to use artillery in that situation, especially since none of the Israeli troops was injured by the Hamas shooting. He replied that “the question is whether or not they were under great or imminent risk.” If they were under imminent risk, he argued, then they might be “allowed to fire artillery or mortar shells into urban areas.” A retired Israeli general makes the same point: “[T]o rescue forces that are getting into trouble, sometimes you have to use a little more firepower.” Another Israeli official says it’s complicated: “Terrorists shooting on our soldiers, our soldiers reacting.”

But this line of thinking loses its relevance as you move from defense to offense. When the enemy is shooting at your civilians, that’s terrorism. When you send troops into the enemy’s territory, and the troops get shot at, that’s not terrorism. That’s plain old war. In Jabaliya, the only civilians immediately at risk were Palestinian. In that context, when you invoke “imminent risk,” you’re no longer using the safety of your civilians to justify the killing of enemy soldiers. You’re using the safety of your soldiers to justify the killing of foreign civilians. And you’re the one who put the soldiers at risk to begin with, by invading.

4. “Human shields.” Israeli briefers have cited many cases in which Gaza militants launched rockets from, or stored rockets in, U.N. facilities. With regard to Jabaliya and other tragedies, the retired Israeli general interviewed by the Times argues that Hamas fighters try to draw fire from the IDF “and hope that some mistake will cause a disaster in order to delegitimize Israel.” Israel’s prime minister and other officials have argued that Hamas’ use of human shields makes it completely responsible for any civilian casualties in Gaza.

This mentality makes it that much easier to pull the trigger. The Times says Israeli officials have offered no evidence that enemy fighters were near the Jabaliya school, and interviews with people on the neighboring streets found nobody who had seen fighters in the vicinity. Nor were there any bullet casings or holes. Does the enemy’s frequent use of human shields justify killing civilians in an instance where there’s no evidence of that behavior? Did this rationale play a role in the IDF’s decision to shoot?

In general, I’m sympathetic to Israel. It’s vastly more careful about sparing civilians than Hamas is, and some of its measures to reduce civilian casualties are exemplary. But you can’t let that blind you to what happened here. The frightening implication of the evidence from Jabaliya is that you don’t have to be evil to perpetrate such a catastrophe. You just have to be human.

Read more of Slate’s coverage of the Israel-Gaza conflict.