Are we prejudiced in legal decisions when we make conclusions about intention since it can't be proven?

Question

Hypothetical: A murders B, and goes to trial. A gets convicted and receives the death penalty. We assume his intention was nefarious (it was also illegal--which is why he's being punished). We assume in part that he did it because he is a low life murderer indiferent to the value of others' lives,, so his intention was evil and thus we believe retribution is a just sentence. A is executed, and some people feel better. They couldn't murder him on their own because it's illegal, but the state does. Everyone who feels good about the execution feels the killing proved justice prevailed. But how can we know their intention for wanting the guilty party is a desire for justice. Maybe they are simply bloodthirsty deviants who enjoy watching someone die & this gives them the opportunity. You can't prove it either way because it goes to intention, which can't be proven, and whatever they say, they might be lying. So all things being equal, doesn't the death penalty rest on the assumption that the original killer's action is far worse than the punishment given to him/her even though it's not provable -- (unless maybe the original murderer admitted as much). Again hypothetically, if the prosecutor argues that one of the reasons he wants the convicted murderer executed is because he, the prosecutor, enjoys seeing people die, in fact, its his favorite pastime, can we assume this should have no bearing on the sentencing because again, it's a legal issue, and that part of the intention on the part of the prosecutor is irrelevant, even though it may be unpleasant--assuming it doesn't cloud his prosecutorial abilities.

Answer 1

You are beating a dead horse.  This has been discussed since Cain killed Abel.  Was Cain justified, was his punishment just or should he have received the death penalty?  Did he have criminal intent?  

Answer 2

Some crimes are so heinous the best thing we can do is send the person to God for judgement.

Answer 3

We have an adversarial process that requires the prosecution to convince 12 members of the public of each element of every crime beyond all reasonable doubt.

You may "assume his intention was nefarious." However, a jury must assume that the accused is innocent until proven guilty.

In most cases, even the prior bad acts of an accused are not admissible to prove that the accused is guilty of the present crime. I watched a child-rape trial where the defendant was acquitted for climbing through a window to commit the rape. The jury was not advised that the defendant had, on two prior occasions, climbed through a window to commit rape.

You use the term "proof" pretty loosely. Proof is a conclusion arrived at by the persuasion of evidence. I can't "prove" the existence of gravity, but I can show you a lot of evidence that gravity exists.

And no prosecutor would ever argue for the death penalty on the basis that s/he gets his or her jollies out of seeing people put to death, any more than the Supreme Court would pass down a ruling on the basis that "we just hate citizens."

The death penalty is employed far more for plea deals than it is for actual executions. In that regard, at the minimum, the death penalty is very effective.