Acts chapter 9

What else had Saul been doing besides "breathing threats and murder against the disciples of the Lord" (Acts 9:1)?
"As for Saul, he made havoc of the church, entering every house, and dragging off men and women, committing them to prison. Therefore those who were scattered went everywhere preaching the word." (Acts 8:3-4).

Why did Saul want to go after the Christians in "Damascus" (Acts 9:2)?
Only "the apostles" (Acts 8:1) were left in Jerusalem, so finding Christians to arrest in Jerusalem may have become slim picking.

Why sort of "letters" (Acts 9:2) do you think Saul sought from the high priest?
Letters of introduction that would serve as arrest warrants authorizing him to arrest Christians and "bring them bound to Jerusalem" (Acts 9:2).

What was the name of the "high priest" (Acts 9:1)?
Given that he would have issued an official letter to introduce Saul, it is likely to have been Caiaphas, the official high priest, rather than Annas, his father-in-law and former high priest who had been pushed out by the Roman but who still wielded power behind the scenes (see Annas and Caiaphas).

What do you think the high priest thought of Saul?
He most likely couldn't say enough nice things about him.

What did Saul abound with?
Zeal, passion and sincerity in what he believed.

What didn't he have?
The truth

What did that combination lead to?
"Murder" (Acts 9:1), like the ones the zealous Jews committed against Jesus (see Crucify Him) and Stephen (see Stephen stoned), those that zealous Roman Catholics committed during the Inquisition and the Crusades, and those committed against Christians by zealous Muslims today.

Does zealously and sincerely believing in something make it true?
 

What is the only infallible source of the truth?
The Bible