What is suspicious about what the guards did to Tom Robinson?

It is suspicious that the guards felt it necessary to shoot Tom Robinson seventeen times.

“Oh yes, the guards called to him to stop. They fired a few shots in the air, then to kill. They got him just as he went over the fence. They said if he’d had two good arms he’d have made it, he was moving that fast. Seventeen bullet holes in him. They didn’t have to shoot him that much.