Occasionally, in order to secure a conviction (which can be very difficult), prosecutors will make a deal with the accused that they will only be charged with a lesser offense if they agree to plead guilty.
In the event that an observant Jew has been charged with a particularly egregious crime ר”ל is there a formal/technical concern about chilul Hashem in “admitting” to such crime when one is innocent? Is it possible to speak about this generally or does it depend upon the particulars?
It would be permissible to make a plea bargain as it is common knowledge that this is a legal tactic and often does not reflect the truth. The defendant and his legal team will often state this openly. The technicality of entering the plea does not necessarily increase Chilul Hashem. In many cases, the charges alone and their being spread by the media cause the greatest Chilul Hashem, regardless of the outcome and trial.
As you mentioned, however, this question would certainly be case dependent, and if in fact making a plea would cause a great Chilul Hashem, it is something to be considered. This would depend on the standing of the defendant in his community, the nature of the charges and the evidence against him.