Another note that might help: To plead a defendant, used as a transitive verb, is insider court jargon, meaning more or less have them plead, get them through the process of a plea.
Without a little more context I'm not sure whether it means merely (1) to enter a plea on their behalf before the court (i.e., plead guilty, no contest, or innocent), or (2) to strike a plea bargain on their behalf; essentially, to assume they're going to have to plead guilty and agree to a (hopefully lesser) charge in order to avoid trial.
To me (2) is possibly more likely because it's more cynical, but even (1) would express an assembly-line procedure for getting people into and out of court fast. Public defenders don't have a great reputation. Some sincerely try to help but often burn out from being underpaid and grossly overworked, and often unable to prevail against the entrenched system. Others are just not very good lawyers who work essentially as cogs in the system, just getting the paperwork done with the minimum required effort, not actually standing up for clients' rights at all.
The latter type seems to be what the accusation was about -- he was afraid that if a client thought he didn't work hard enough to defend him, the client might attack him or have someone else attack him, so he bought the gun.
Not sure how to put any of that into German -- I agree with dude's overall assessment, but it would help if someone could actually translate the other words.