Dead flies. Hebrew flies of death: i.e. flies that bring or produce death. Supply the Figure of speech Ellipsis, "[as] dead".
cause = [are that which will] cause, &c.
to send forth a stinking savour = to stink [and] fer-ment. Figure of speech Hendiadys.App-6.
folly = stupidity. Hebrew. sakal. Same root as in Ecc_10:6. See note on Pro_1:7.
him. Note the Figure of speech Ellipsis (App-6): "So doth stupidity [cause] him that is in reputation for wisdom and honour [to send forth an offensive savour]".