1. A belief that courts, and not news outlets, are best placed to determine a person's guilt. Consider how you, personally, know whether or not this person did anything. You saw it on CNN? Do you really think news outlets are reliable enough to base executions on?

2. Respect for "innocent until proven guilty". If you don't believe in that, imagine being falsely accused of paedophilia, or plotting to commit an act of terrorism, on the say-so of someone with a grudge against you. Do you really want mob justice? If so, pray the mob never turns against you. And read the stories posted by commentors on the page linked below.

3. The fact that an adversarial system, such as Anglo-American legal systems have, simply cannot work without a defence for accused persons. Any mitigating factors (such as mental illness) will be presented to the court and duly considered. If they're ill-founded they'll be rejected. If they're well-founded then they'll be rightly taken into account. Of course courts aren't perfect and don't always get it right, but they're a hell of a lot more reliable than emotion-fired mobs.

4. The fact that if he isn't allowed a defence lawyer, he can't be tried, and therefore can't be convicted. Nobody can be forced to defend themselves against expert prosecution lawyers – people who defend themselves have chosen to do so. This follows from point 3.

Accordingly, the kindest thing for the victims' families is for the lawyer to defend the accused as well as she can, because if she doesn't do her best job there's the risk of a mistrial followed by years of appeals, denying closure to the victims' families.

5. The need to establish a person's mental state, as well as their purely physical actions, as part of criminal guilt or innocence. Mens rea (the "guilty mind") is an absolutely basic and fundamental concept to the criminal law. If you get rid of this, you'd execute a devoted husband who kills his wife while sleepwalking and dreaming that he's being attacked by an armed man, or while high on LSD which someone else put into his glass of water without his knowledge.

6. Opposition to the death penalty might be a motivation, but personally I think the other arguments are quite enough.

7. The love of a challenge. Given that somebody has to take on this difficult task, a person who enjoys taking on difficult tasks might decide it might as well be them.

8. Don't forget that defending an accused person doesn't necessarily mean denying that they did it. I expect this accused will admit the killings but plead insanity, and diminished responsibility in the alternative. (Not strictly speaking a reason, but an important point nonetheless.)

I think that's enough to be getting on with, don't you?