A human being is not a machine. What people seem to expect is that a victim of armed robbery should be able to act in a perfectly rational manner. No consideration is given to the fact that the pharmacist would be likely experiencing an extreme emotional distress by being put into this situation. And unless people have been in that situation themselves, who are they to judge?
The principles of legal self-defense go back hundreds, maybe even 1,000 years to English Common Law. They aren't some technicality some liberal dreamed up in the 1960s.
If Ersland wanted to testify that he was operating under extreme emotional distress, he could have done so. The judge granted a lesser included offense that would have allowed for such mitigation.
Ersland made no such claim; you are entitled to make it for him in a discussion such as this one, but the jurors did not legally have that option.
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What if Ersland had killed several innocent bystanders in the street? (He came very close to doing so.) Would you still argue that he was acting under distress and nobody had a right to judge him?