He could have easiyl killed Shirou a hunderd times before
And while he is commanded to not attack Shirou, he doesn't really need that seal lifted. All he has to do is let Shirou get killed on his own or goad another servant into doing it for him.
There was nothing stopping him from letting someone else kill Shirou and he could just then murder everyone. It's not like Caster can stop his surprise attack.
The simpler answer is that he really doesn't want to kill Shirou, just get him to snap out of his self-destructive world-view.
Attacking him at the temple, from behind - he could have easily scored a fatal wound, but he didn't.
Like with Caster, what he DOESN'T do tells a lot.