Well, if they killed him, they probably wouldn't be able to get the keys. And they probably had to keep the bigger "punishment", imprisonment, looming over his head in case he reveals confidential information about the case.
Once you've got this new cert. you can MITM, but you can't use it to decrypt the traffic already captured. Also anyone paying attention sees the cert. fingerprint change out of the blue.
B) Anyone who knows what they're doing and has something they really want to keep secret? Maybe if someone had such a secret they'd learn to check the cert, maybe even install an extension that would highlight unexpected changes.