This isn't a criminal trial, this is representative government.
It's literally an investigatory panel.
It would be utterly indefensible to place those under investigation on the panel investigating their own behaviour, handing them the power to let themselves off the hook. It would completely delegitimise the process, reducing the likelihood of trust and restitution to zero.
That is not a principle which exists only in a criminal trial. It is a principle which exists pretty much universally. Just imagine experiencing improper conduct from a colleague at work, going to HR, and HR placing that same colleague in charge of your complaint.
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Here in the UK, there is currently a high-profile investigation ongoing into a string of parties that took place in 10 Downing Street and a few other government buildings. There is widespread public anger, because it appears very strongly that the Prime Minister held numerous booze-ups when the rest of the country was in full lockdown.
The government agreed to hold an independent inquiry. It appointed the Cabinet Secretary Simon Case to lead it. Then a short while later, it turned out that one of the parties had been held
in Simon Case's office.
You know what happened when that become public knowledge? He recused himself. Because there is absolutely no fucking chance in hell that anyone in the country would respect the outcome of an inquiry into wrongdoing
led by someone who was himself implicated.
And no, it's not a fucking valid defence of this to say that the opposition drove him to drink in the first place, so the opposition is just as much to blame.