User blog comment:FairyRaven/Fairy Tail Chapter 392: You Must Not Forget, Review and Commentary/@comment-7418318-20140711094725/@comment-7418318-20140711111534

Maybe Keith isn't literally controlling him like a puppet on a string, but controlling his actions by implanting orders in his mind that Silver can't resist. More like a conditioning.

As long as Silver isn't violating his orders and compulsions he can act on his own, but he is unable to disobey this orders.

That would explain all the stuff I couldn't find a reason for. If Keith for example ordered Silver his best to make Gray dispair, it'd would explain why Silver would be so cruel to his son.

Though that would leave the question of why Keith would ever bother with Gray? Could Doriate have been Keith's subordinate and now the necromancer want revenge for that?