1Disclaimer: I do not own Star Wars
You never found it odd, your frantic obsession for the past two years. You never found it odd because you always had a good reason.
The reason all led up to eventually getting rid of him. You don't deny it; it's the way of the Sith after all. Any remaining loyalty to your master has long since disappeared, to be replaced with a cold hatred. He's just in your way now, another obstacle to be rid of just like the rebels. If only he was as easy as the rebels.
That's why you never found your obsession weird. That's why you didn't care when your murderous rages became more and more frequent. Officers can be replaced, the boy cannot. Because the boy is the only option. As much as you hate to admit to yourself, you are no longer near as powerful as you once were. And you cannot overthrow the emperor alone.
It does it occur to you that there are other force-sensitives out there, that Skywalker's – since when did you start calling him Skywalker? – not the only one. But you know that Skywalker's the strongest, his presence is like a brilliant sun shining in the Force. It gave you some satisfaction when the Emperor ordered you to kill Skywalker – you knew he was scared. It gave you even more satisfaction when he gave to permission to turn Skywalker. You rarely gloat anymore – you learned your lesson from your younger days – but you couldn't help smiling triumphantly behind your mask then. The emperor had practically given you permission to overthrow him.
Rarely you do you think on how you had first started to truly hope that you could succeed in killing Palpatine when you heard of Skywalker. But the thought hardly ever occurs to you, and when it does it confuses you because you have no answer as to why this is. You always shrug it off without a second thought.
Because you are sure of your reason.
But as you stand on the bridge of the Executor, watching the Millenium Falcon and your son disappear into the stars – they're probably on the other side of the galaxy by now – you notice something else. Perhaps it's because you know you'll never get an opportunity like this to convert your son ever again – the Emperor will be more careful from now on. Perhaps it's the deep bitter disappointment filling you at the loss of your son and future apprentice. Perhaps it's that slight ache in your chest that you never noticed before and that you're sure wasn't caused by any injury. Perhaps it's the surprise at your son's absolute refusal – even to the point of jumping of the balcony on Cloud City – to join you.
Whatever it is, you begin to think that you may have had another reason for the last two years. Luke may have been your reason.