By now, nothing Ramsay does on Game of Thrones can shock us, and even now, no one is really sad about it, but we can't help but wonder why Ramsay killed Roose Bolton? The short answer is, easily, to get him out of the way so that he could be the one to lead their soldiers to get rid of the last of any remaining Starks. Which, as we know, includes Sansa and Jon Snow, because they're the last people even remotely standing in the way of Ramsay totally owning Winterfell.
It's also plenty worth it to mention that Ramsay killed Roose Bolton just moments after learning that his wife had just given birth to Roose's second son and, in a way, only true heir. Yes, Roose had already had Ramsay legitimized, but come on. Nothing beats the real deal in the Seven Kingdoms. We all kind of expected Ramsay to kill his stepmother way back when it was found out that she was pregnant, since any legitimately born son would threaten Ramsay's chances of ruling as Lord Bolton, period.
But to plunge a surprise knife into Roose's gut, after even being told how important he is as a son, was shocking even for Ramsay. Then again, he probably would have a hard time getting away with the murder of the rest of his "family" if Roose was still alive and well. And how would Ramsay be able to rule over Winterfell, flaying anyone who pissed him off in the slightest, if his dear old dad was still around to control him? Roose Bolton was the only person who had at least some kind of influence or control over Ramsay and with him gone, it means that Sansa's psycho husband can have free reign to be just that.
So basically, Ramsay did what we all expected him to do from the very beginning, and killed his father to gain control. Not that he wasn't already doing whatever the Hell he wanted in Winterfell. I.e., torturing Sansa and Theon, respectively. But considering the kind of man Ramsay is, it's no surprise that he killed Roose, and then quickly thereafter did away with his dear stepmother and new baby brother.
Ramsay killed his father because he's Ramsay of course, but also because doing so made it possible for him to be the Lord in charge and be able to lead his army on the whole of the north.