Where do you stand on the Hussain-Inglis spectrum? The former is Munir Hussain, the householder who chased a burglar down the street and then beat him unconscious with a cricket bat – and who was released from jail this week after his sentence was reduced by the Court of Appeal. The latter is Frances Inglis who was, on the same day, sentenced to serve a minimum of nine years behind bars for the "mercy-killing" of her teenage son who had been in a kind of coma for three years.

There is something iconic about these two cases, but something political too. For they play upon two very different sets of prejudices and perceived rights.

On the one side is the right of the householder to protect his property. Here the protagonist was chairman of the Asian Business Council and his victim a career criminal with more than 100 previous convictions.

But after breaking into the Hussains' home, and threatening to murder its occupants with a knife, the burglar, Walid Salem, was pursued down the street by Mr Hussain and his brother and hit so hard that he suffered a fractured skull, ribs, jaw, and finger and brain damage. He was so badly injured that he was deemed unfit to stand trial.

Though an Englishman's home may be his castle he doesn't have the right to pour boiling oil on those violating it. Violence by the homeowner has to be proportionate and reasonable, and the court decided that this wasn't. To add further spice to the mix Walid Salem, whose lawyers claimed he was unfit to plead, was then alleged to have subsequently committed other offences.

There was much rejoicing in certain quarters this week when Mr Hussain's 30-month jail term was reduced to a suspended sentence and he was freed.

By contrast the other citizen convicted of taking the law into her own hands this week is now beginning a minimum stay behind bars of nine years. Frances Inglis gave a lethal heroin injection to her 22-year-old son Tom, who was so brain-damaged that he was unable to speak. He could communicate only by blinking and had to be fed through a tube into his stomach.

Mrs Inglis, who was a community support worker for adults with learning difficulties before starting nursing training, was described by an Old Bailey judge as a "devoted mother highly regarded for [her] work in the community". But, he added: "You knew you were breaking the law, and you knew the consequences."

Justice is political as well as principled. That much was clear from the extensive support voiced for these two representatives of the middle classes when they appeared in the dock. There was more support for Mr Hussain from the right of the political spectrum. And there was more indignation about the length of Mrs Inglis's sentence from the liberal-left. But the faultlines are more interesting than the usual polarities.

It would be a mistake to expect the debate to be governed by logic, of course. Those of a right-wing disposition clearly place different value on the life of the householder and that of the burglar, much as in other debates they see the "sanctity of life" as absolute for unborn children but less so for convicted murderers. Other determinants of moral worth evidently come into play. The implicit assumption is that a burglar, by breaking the law, forfeits his right to be protected by it. There is a dangerous double standard in that.

Then, on the other side, no proper rationale is advanced by the pro-euthanasia camp as to why one individual's "right to die with dignity" ought to supersede the right to life of a sick person whose relatives are simply tired of caring for them, or who see profit in their death. There is just the utilitarian assumption that there will be more of the former than the latter, which is a very poor justification for ignoring the rights of a minority.

The real decisions in such matters are about how we balance the rights of one individual with those of others in society, particularly its more vulnerable members. It is a debate about achieving an equilibrium between personal freedom and the common good.

Part of the point of law is that it seeks to protect us all from the judgement of self-righteous individuals, or indeed political pressure groups. That is why it says it cannot be for the householder to decide what constitutes "reasonable force" in defending their home – and a jury decided that breaking a cricket bat in three pieces over someone's head was not reasonable. And it is why it insists that no parent, however loving, has the complete say over the right of their child to life.

Frances Inglis decided that her son would not want to live the life he was leading. She said at her son's hospital bedside "all I saw was horror, pain and tragedy". But society is open to balance that against other judgements, like that of one of the doctors at her son's hospital who offered a more encouraging prognosis. "How can they know how he feels," she wrote. The court decided that the same question could be asked of the trainee nurse. It was far from a clear-cut case of consensual assisted dying.

All this also goes to the question of what punishment is for. Punishing anyone is wrong, unless they are likely to learn from the experience, one commentator opined yesterday. That might be so were the only point of prison preventing the criminal from repeating their crime. The chances of Frances Inglis "doing it again" are next to zero. And Munir Hussain is hardly a danger to society, apart perhaps from those small sections of it who chose to break into his home. But prison sentences serve another function; they act as a deterrent to others who might be contemplating a similar crime.

The business of contemplation is an important determinant. In both the Hussain and Inglis cases those in the dock had demonstrated a loss of control. The Asian businessman's attack on his assailant was wild and frenetic. And Mrs Inglis was, a friend testified "mad and so upset – she couldn't be consoled"; after she had injected her son with the fatal dose, she barricaded herself in his hospital room and shouted to staff that she had HIV, threatening them.

But Mr Hussain was found by the Court of Appeal, even in the street after the burglary, to be "acting under the continuing influence of extreme provocation". By contrast Mrs Inglis had clearly premeditated the killing of her son, carefully planning it even while she was on bail for a previous attempt at injecting him with a fatal overdose. She acknowledged this, but tried to side-step it by saying: "The definition of murder is to take someone's life with malice in your heart. I did it with love in my heart, for Tom, so I don't see it as murder." It is not a distinction the law recognises, and nor necessarily should it.

The key point it that its not for individuals to make such judgements. It is for society to do that, a function we delegate to a jury and a judge. There were cries of "shame on you" at the jury in the Inglis case when they delivered their verdicts. They came from weeping members of the dead boy's family, watching from the public gallery. Such a reaction is understandable but wrong. The jury had, as the judge instructed, "put all emotion aside" and that was right.

Victims are not always best qualified to judge when justice has been done. Though it is today fashionable to talk about the voice of the victim being heard in court, that process must have limits. Distancing victims from the judicial process is what distinguishes a revenge culture from the civilised rule of law.

It was revealing that Michael Wolkind QC, who defended Munir Hussain, yesterday said that he does not agree with Conservative party proposals to give added rights to householders to defend themselves. "The test at the moment is that the defendant behaved reasonably," he stated, "and that should be good enough."

Significantly the Court of Appeal did not overturn the verdict against Mr Hussain, Rather it determined, in the words of the Lord Chief Justice, Lord Judge, that the case was "truly exceptional" and adjudged that the original sentence passed upon him had been disproportionately severe.

Appeal judges may come to a similar conclusion in the case of Frances Inglis. The judge in her case mitigated her sentence, on compassionate grounds, down from the 15 years which is the starting point for sentencing in a life sentence, to nine years. Perhaps the Court of Appeal will decide he could have gone further, and reduce her sentence to three or four years.