He kidnapped the Queen clan and tried to force a tied-up Oliver to choose between his mother and his sister.
The murder of Moira Queen goes down as one of the biggest tragedies of the first two seasons of Arrow, so it’s no surprise that Stephen Amell considers Oliver’s inability to protect her as Oliver’s biggest regret.
Moira volunteered herself so that Thea would survive, and both her kids had to watch Slade skewer her with a ginormous sword.
Slade Wilson really did everything he could to make Moira’s death as horrible as possible.