I can't think of any scriptural grounds for someone not to be a judge. This would be a conscience matter. There are cases of witnesses still being police officers, too, even if it is not very common.
There are witnesses who own guns to hunt as well, although it's not common in the mainstream.
And you can't cherry pick out cases this guy may have presided over to somehow question his who he is. People's lives are not perfect yet somehow we act like all witnesses have to live only one type of life and if they are not perfect they are hypocrites. I personally am a work in progress and view everyone else that way too. I see ex-witnesses sometimes are as judgmental as witnesses and many times look for things to be wrong, because they want it to be that way.
None of us know this guy personally. There are enough bad people that we don’t have to make EVERYONE bad because of our emotional response to every witness.