Why would I do that?! I did not want to look like a fool but now look what happened.
Coming into encounter with the elephant who has just killed a coolie. The person who Orwell to go kill that elephant left, once he went to do so a crowd of 2,000 people formed to watch. Knowing that the elephant is not a danger he does not want to kill it, but being that the crowd is watching, if he were to walk away he would look like a fool.
With firing at the elephant ultimately hurting it, he felt the hurt the elephant is feeling and is being mentally drained. Watching the elephant in pain is only causing him more mental distress, so he tries to figure out where he can shoot the elephant to kill it instantly. In doing so he hopes to end its misery and slow and painful death.
The aftermath of the shooting led to him facing the mental oppression in his actions. Being that the elephant was not of harm to anyone anymore and he did not want to do it, it took a mental toll on him. Yet, facing the crowd as a fool was going to do the same as well. He was stuck between two lines with no way out.