How did he know that would be the consequences of his actions when plenty of people have been sentenced to life in prison for similar actions? How did he know he wouldn’t be dealing with a hung jury? Unless he was able to predict the future, which he could not, suggesting he knew that ‘capital punishment by lethal injection’ would be the consequences of his actions is ridiculous. On top of that, he may literally not know the difference between right and wrong, something that is entirely possible. In which case, again, he would not have known the consequences of his actions.
And it’s not about his feelings. The law shouldn’t be sidestepped just because a crime is very bad. Otherwise, why not just let police kill people like him and avoid trial completely?