What you have to understand is that every case is different. Ten people charged with the same crime will have a different set of facts in each one. Any plea offer made by the State and accepted by the defendant must be accepted by the court. While we as attorney's dislike when judges start asking questions about the facts of a case to determine whether the plea offer is suitable, we all understand they are completely within their right and duty to do so. I know it is tempting to try and compare a harsh sentence on a seemingly minor crime like DUI to a light sentence on murder or child molestation but every case is different with a different set of facts. While a court may take the stance they are "tough on crime," there are exceptions to every rule. Because someone gets what they consider a harsh punishment on DUI and someone else gets a seeming light sentence on a more serious crime the two cannot be compared. If a judge is doing his job, the outcome in any case should be a fair resolution for both parties, both the State and the defendant. That doesn't mean everyone will agree with the result. It just means that that justice is different in every case. As a society we task our judges to determine what is fair and just. i hope this sheds some light on your concerns.
I doubt the judge lets "killers" into ARD. That requires the DA to agree and I doubt DA s are letting "killers" into ARD. Moreover, you can't even get ARD for murder. As for the judge rejecting the plea agreement, I am going to speculate here that the DA offered to let the person in as if he had a lower BAC than he actually had. I see that arrangement rejected by a lot of judges. House arrest for a fourth offense is also illegal but I have obtained such a sentence for my clients but I have also had such a deal rejected by judges.
If your friend and his attorney feel that the judge is prejudiced against persons accused of DUIs, you may be able to challenge the judge for his bias and have him either recuse himself or have the case sent to another department (another judge). You should consult with an attorney as to what factors must be shown for such a challenge and whether or not you still may challenge the judge.