There are two ways to answer this question. First, from official story of the guards, Tom was trying to escape. They stated that he was fast, and if he had two good arms he would have escaped. They also stated that they first shot into the air a few times to warn him, but that these shots were not heeded. This storyline says that Tom did not listen and that he was possibly dangerous. The point is that the shots were warranted.
The other way way to answer this question is to underline the racist nature of the guards. Tom was not a human being with rights, but another black man. In their eyes, the world with one less black man was a better world. Here is what Atticus says:
“Depends on how you look at it,” he said. “What was one Negro, more or less, among two hundred of ‘em? He wasn’t Tom to them, he was an escaping prisoner.”
As a reader it is hard not to view racism all over the place.
It is also important to note that the guards probably felt no sympathy for Tom, since he was black. In a sense, it was disrespecful for them to have treated him so brutally, even if he was trying to escape. Had he been a white prisoner, they probably would not have shot him so many times, though he might still have been shot trying to escape.
I agree with both, because most would agree that one shot would have done enough damage to anyone, let alone a crippled man. Obviously the guards were racist because two shots is one too many.
Tom was shot 17 times, because they guards at the camp thought he was trying to escape. They said he was almost over the fence so they began shooting at him. Before this in the trial we discovered one of his arms is almost disfunctional and twelve inches shorter then his other arm. There is almost no way for Tom to climb the fence with an arm like that. The guards had to make up a reason to kill him since he was black.