He figured that having Harry help with the council's counterattack was more important than saving one child that was probably already dead or turned (from his point of view). Also, couldn't he just hide behind a few Wards in the Nevernever if self-preservation was his goal?

And don't we have a Soulgaze confirming he's a good guy (Also, why would being the Blackstaff make him automatically evil)?

Ah but if the council's attack was so important to him why did he leave it. If it was me I wouldn't take the chance on the curse working Ortaga's widow was putting it together specifically for him don't you think she would have thought about him doing just that and taken precautions against it.

The soul gaze isn't a lie detector and it's a one shot deal people change besides that EB might think he's perfectly justified in forming the BC like Jim often says people rarely think of themselves as the monster.

Being the black staff doesn't automatically make him evil it just means he is willing to violate everything he claims he believes in to further his and the council's agenda if that wouldn't  screw up the sanest of people I don't know what would.