Even though you are communicating the action that happened after Boo's encounter with Bob Ewell, you do not necessarily have to use a past tense monologue. You could have Boo describe the events as if it were happening at that moment.
"I am watching the children pass through the darkness, and I see a man stumbling after them. I wait until they nearly pass me, and then I see that he is about to attack them. I move forward..."
You could create a monologue with Boo giving a statement to Sheriff Tate where he describes what happened from his point of view. I think a past tense monologue would be easier, but a use of present tense might be more interesting and challenging.