
Wouldn't be sure about that. Let's consider the tragedies that happened to him,it's not surprising for him to change like that. If someone can make the people who made his life miserable, it will be normal for him to change radically if that's what it takes to do everything he needs,a human's defence mechanism.
-
Right now,I think I'm more interested in the connection between Undertaker,Claudia(Vincent's mother) and Cedric K. Rose (Claudia's legal husband according to the Ofiicial Phantomhive Family Tree).

It's just the way that Undertaker approached Ciel not to mention what we saw of his flashback during the Green Witch arc.
It hints that there are things buried in Ciel's mind. Be it memories or personality traits or so forth. Something is hidden and the Undertaker might have hidden it...or Sebastian did and Undertaker wants to wakes it back up.

Oh God I'm really in love with the concept of breaking the contract!! XDD
-
I'm also thinking that's their connection but it would be too typical if that happens and we all know that Yana-sensei is not a fan of it too so she'll make their connection something that will surprise us. I'm really excited on how she will develop the characters~!!
Is Vincent inside Ciel?!! Undertaker is unusually obsessed with Vincent..