Legitimately is basically any reason other than a categorical opposition to the death penalty. Again, the imposition of the death penalty is almost entirely discretionary. So unless you have jurors going into it before hearing the evidence saying "there is no way I will ever impose the death penalty," any decision is legitimate. Jurors are instructed to basically search their conscience and come to a decision they feel comfortable with -- for any reason.
Neither one of us were in the jury. From what I've heard in the media, I personally think she deserved the death penalty -- especially because of the lack of remorse she displayed (she seems like a manipulative viper, to put it nicely). But she put on a real sob story for the jurors about how she was abused and has mental problems, so who knows what they heard, what they deliberated about, and what they were thinking. I'm not in a position to second-guess, and really neither is anyone else except the other jurors and people who were actually at the trial and heard all of the evidence. As I'm sure you know, the media is not super reliable when it comes to presenting balanced coverage.
Here are some sample jury instructions on death, to give you an idea:
By law, you must now consider whether justice requires imposition of the death penalty, or life in prison without any possibility of release, on the defendant.
This is a decision left exclusively to you, the jury. I will not be able to change any decision you reach regarding the death penalty, nor regarding life imprisonment without any possibility of release. You, and you alone, will decide whether or not Defendant should be executed or sentenced to life in prison without any possibility of release. Thus, I again stress the importance of your giving careful and thorough consideration to all evidence before you. I also remind you of your obligation to strictly follow the applicable law.
At the same time, I remind you that none of you individually, nor the jury collectively, is ever required to impose a sentence of death.
In short, the law does not assume that every defendant found guilty of first degree murder should be sentenced to the death penalty. The law does not assume or presume that Defendant, as he sits before you, should be sentenced to death or to life in prison without any possibility of release.
This decision is for you, the jury.
In order for you to decide that Defendant should be sentenced to death, you must reach a unanimous decision that the government has proven beyond a reasonable doubt that death is the appropriate sentence. This means that each of you must be individually convinced, beyond a reasonable doubt, that death is the appropriate sentence before it is possible to sentence Defendant to death.
If just one of you believes that death is not appropriate – or that the government has not proven beyond a reasonable doubt that death is the appropriate sentence – then you must not sentence Defendant to death, but rather to life in prison without any possibility of release.
You must each decide whether the sum of the aggravating factor or factors you have unanimously found to exist, sufficiently outweigh the mitigating factor or factors that you have individually found to exist -- including the mitigating factors to which the parties have stipulated -- so as to justify sentencing Defendant to death.
This determination is left entirely to each of you, individually.
If you have decided that the aggravating factor or factors which you have unanimously found to exist, sufficiently outweigh the mitigating factor or factors which you have individually found to exist -- including the mitigating factors to which the parties have stipulated -- you must then decide whether to impose death.
You are never required to impose a sentence of death.
In carefully weighing the various factors at issue in this case,
you are called upon to make a unique, individualized judgment about the propriety of sentencing Defendant either to death or to life in prison without any possibility of release.
This is not a mechanical process. Rather than counting factors, you should consider them qualitatively.
Your decision must be a reasoned, moral response.
You are never required to impose a death sentence. You have
complete control and discretion in determining whether or not the circumstances of this case justify a sentence of death.
You must consider whether the aggravating factor or factors you have unanimously found to be established in this case sufficiently outweigh the mitigating factor or factors each of you, individually, find to be established -- including the mitigating factors which both parties have stipulated exist -- before you may consider imposing a sentence of death.
You may consider mercy in making this determination.
If you decide to impose the death sentence, the Court is required to impose that sentence. If you decide that the death penalty should not be imposed in this case, Defendant will be sentenced to life in prison without any possibility of release.
...
.