In addition, Walter says the monster is trying to kill him; That relates to the fact that Walter submerges a spoon in his neck two inches. Of course, Valtiel isn't just gonna pop out of nowhere & pwn him in prison. However, we know for a fact Valtiel is inside Walter's mind.
I presume Valtiel decided to stop Walter because either his murder spree was getting out of control or he was going to choose an insufficient vessel, Apt. Room 302. However, apparently Valtiel failed to do so because he is seen entering Apt. Room 302 a few days later . . .