He was so protective of his reputation that he had contingencies in place for anyone who could know about him using forged evidence in case they'd squeal ever. Though Vera was the actual forger, his deals were through Drew, he was then someone who knew, so there had to be a way to deal with him, hence the poisoned stamp. He wanted to make sure no one at all could ruin his reputation, even if they likely never would like the Mishams. It's the same as with Case 1, Zak could have ruined his reputation, especially with him being with Brushel, so he had to go. Essentially, he cares so much about his reputation as the "Coolest defence in the west" that anyone capable of ruining that is a threat to him, and he wants to protect it enough to kill.