Basic explanation: if they don't think he's a flight risk, don't think he'll mess with evidence or witnesses and they don't think there's a risk of him committing another murder in the meantime before a trial, then it's reasonable to grant bail. When you consider the cost of keeping someone in jail before their trial (and remember, it can potentially be years) and the fact that at this stage, he isn't guilty of anything, then ensuring only the most dangerous people, or those who are likely to run, or those who are likely to intimidate witnesses/etc are kept in jail pre-trial.
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