Something of a nitpick, but you can believe one person is guilty pre-trial without thinking everyone is - there simply needs to be enough evidence to pass your own burden of proof (there's no obligation on a person, outside of a jury, to uphold the 'beyond reasonable doubt' standard), or that you believe there is no room for reasonable doubt even at this stage.

A person unconnected to the case commenting online has far less of an impact than a court with the power to impose criminal sanctions, so using a lower standard of proof isn't particularly harmful.